Boolean is an instance of PrimitiveType. In the metamodel, Boolean defines an enumeration that denotes a logical condition. Its enumeration literals are: . true - The Boolean condition is satisfied. . false - The Boolean condition is not satisfied. An instance of Integer is an element in the (infinite) set of integers (..2, -1, 0, 1, 2..). It is used for integer attributes and integer expressions in the metamodel. A string is a sequence of characters in some suitable character set used to display information about the model. Character sets may include non-Roman alphabets and characters. An instance of String defines a piece of text. The semantics of the string itself depends on its purpose, it can be a comment, computational language expression, OCL expression, etc. It is used for String attributes and String expressions in the metamodel. An unlimited natural is a primitive type representing unlimited natural values. An instance of UnlimitedNatural is an element in the (infinite) set of naturals (0, 1, 2..). The value of infinity is shown using an asterisk ('*'). VisibilityKind is an enumeration type that defines literals to determine the visibility of elements in a model. Semantics VisibilityKind is intended for use in the specification of visibility in conjunction with, for example, the Imports, Generalizations, Packages, and Classifiers packages. Detailed semantics are specified with those mechanisms. If the Visibility package is used without those packages, these literals will have different meanings, or no meanings. A public element is visible to all elements that can access the contents of the namespace that owns it. A private element is only visible inside the namespace that owns it. A protected element is visible to elements that have a generalization relationship to the namespace that owns it. A package element is owned by a namespace that is not a package, and is visible to elements that are in the same pack-age as its owning namespace. Only named elements that are not owned by packages can be marked as having package visibility. Any element marked as having package visibility is visible to all elements within the nearest enclosing pack-age (given that other owning elements have proper visibility). Outside the nearest enclosing package, an element marked as having package visibility is not visible. In circumstances where a named element ends up with multiple visibilities, for example by being imported multiple times, public visibility overrides private visibility, i.e., if an element is imported twice into the same namespace, once using public import and once using private import, it will be public. A classifier is a classification of instances - it describes a set of instances that have features in common. Description A classifier is a namespace whose members can include features. Classifier is an abstract metaclass. A classifier can specify a generalization hierarchy by referencing its general classifiers. A behavioral feature is a feature of a classifier that specifies an aspect of the behavior of its instances. Description A behavioral feature is a feature of a classifier that specifies an aspect of the behavior of its instances. BehavioralFeature is an abstract metaclass specializing Feature and Namespace. Kinds of behavioral aspects are modeled by subclasses of BehavioralFeature. Semantics The list of parameters describes the order and type of arguments that can be given when the BehavioralFeature is invoked. Description A feature declares a behavioral or structural characteristic of instances of classifiers. Feature is an abstract metaclass. Semantics A Feature represents some characteristic for its featuring classifiers. A Feature can be a feature of multiple classifiers. A literal unlimited natural is a specification of an unlimited natural number. Description A literal unlimited natural contains an UnlimitedNatural-valued attribute. Semantics A LiteralUnlimitedNatural specifies a constant UnlimitedNatural value. Notation A LiteralUnlimitedNatural is shown either as a sequence of digits or as an asterisk (*), where the asterisk denotes unlimited (and not infinity). A literal null specifies the lack of a value. Description A literal null is used to represent null (i.e., the absence of a value). Semantics LiteralNull is intended to be used to explicitly model the lack of a value. Notation Notation for LiteralNull varies depending on where it is used. It often appears as the word "null". Other notations are described for specific uses. A literal integer is a specification of an integer value. Description A literal integer contains an Integer-valued attribute. Semantics A LiteralInteger specifies a constant Integer value. Notation A LiteralInteger is typically shown as a sequence of digits. A literal string is a specification of a string value. Description A literal string contains a String-valued attribute. Semantics A LiteralString specifies a constant String value. Notation A LiteralString is shown as a sequence of characters within double quotes. The character set used is unspecified. A literal Boolean is a specification of a Boolean value. Description A literal Boolean contains a Boolean-valued attribute. Semantics A LiteralBoolean specifies a constant Boolean value. Notation A LiteralBoolean is shown as either the word "true" or the word "false", corresponding to its value. A literal specification identifies a literal constant being modeled. Description A literal specification is an abstract specialization of ValueSpecification that identifies a literal constant being modeled. A namespace is a named element that can own other named elements. Each named element may be owned by at most one namespace. A namespace provides a means for identifying named elements by name. Named elements can be identified by name in a namespace either by being directly owned by the namespace or by being introduced into the namespace by other means (e.g., importing or inheriting). Namespace is an abstract metaclass. A slot specifies that an entity modeled by an instance specification has a value or values for a specific structural feature. Description A slot is owned by an instance specification. It specifies the value or values for its defining feature, which must be a structural feature of a classifier of the instance specification owning the slot. Semantics A slot relates an instance specification, a structural feature, and a value or values. It represents that an entity modeled by the instance specification has a structural feature with the specified value or values. The values in a slot must conform to the defining feature of the slot (in type, multiplicity, etc.). An instance specification is a model element that represents an instance in a modeled system. Description An instance specification specifies existence of an entity in a modeled system and completely or partially describes the entity. The description includes: Classification of the entity by one or more classifiers of which the entity is an instance. If the only classifier specified is abstract, then the instance specification only partially describes the entity. The kind of instance, based on its classifier or classifiers. For example, an instance specification whose classifier is a class describes an object of that class, while an instance specification whose classifier is an association describes a link of that association. Specification of values of structural features of the entity. Not all structural features of all classifiers of the instance specification need be represented by slots, in which case the instance specification is a partial description. Specification of how to compute, derive or construct the instance (optional) Semantics An instance specification may specify the existence of an entity in a modeled system. An instance specification may provide an illustration or example of a possible entity in a modeled system. An instance specification describes the entity. These details can be incomplete. The purpose of an instance specification is to show what is of interest about an entity in the modeled system. The entity conforms to the specification of each classifier of the instance specification, and has features with values indicated by each slot of the instance specification. Having no slot in an instance specification for some feature does not mean that the represented entity does not have the feature, but merely that the feature is not of interest in the model. An instance specification can represent an entity at a point in time (a snapshot). Changes to the entity can be modeled using multiple instance specifications, one for each snapshot. It is important to keep in mind that InstanceSpecification is a model element and should not be confused with the dynamic element that it is modeling. Therefore, one should not expect the dynamic semantics of InstanceSpecification model elements in a model repository to conform to the semantics of the dynamic elements that they represent. When used to provide an illustration or example of an entity in a modeled system, an InstanceSpecification class does not depict a precise run-time structure. Instead, it describes information about such structures. No conclusions can be drawn about the implementation detail of run-time structure. When used to specify the existence of an entity in a modeled system, an instance specification represents part of that system. Instance specifications can be modeled incompletely, required structural features can be omitted, and classifiers of an instance specification can be abstract, even though an actual entity would have a concrete classification. A structural feature is a typed feature of a classifier that specifies the structure of instances of the classifier. Description A structural feature is a typed feature of a classifier that specifies the structure of instances of the classifier. Structural feature is an abstract metaclass. Semantics A structural feature specifies that instances of the featuring classifier have a slot whose value or values are of a specified type. A namespace is a named element that can own other named elements. Each named element may be owned by at most one namespace. A namespace provides a means for identifying named elements by name. Named elements can be identified by name in a namespace either by being directly owned by the namespace or by being introduced into the namespace by other means (e.g., importing or inheriting). Namespace is an abstract metaclass. A constraint is a condition or restriction expressed in natural language text or in a machine readable language for the purpose of declaring some of the semantics of an element. Description Constraint contains a ValueSpecification that specifies additional semantics for one or more elements. Certain kinds of constraints (such as an association "xor" constraint) are predefined in UML, others may be user-defined. A user-defined Constraint is described using a specified language, whose syntax and interpretation is a tool responsibility. One predefined language for writing constraints is OCL. In some situations, a programming language such as Java may be appropriate for expressing a constraint. In other situations natural language may be used. Constraint is a condition (a Boolean expression) that restricts the extension of the associated element beyond what is imposed by the other language constructs applied to the element. Constraint contains an optional name, although they are commonly unnamed. Semantics A Constraint represents additional semantic information attached to the constrained elements. A constraint is an assertion that indicates a restriction that must be satisfied by a correct design of the system. The constrained elements are those elements required to evaluate the constraint specification. In addition, the context of the Constraint may be accessed, and may be used as the namespace for interpreting names used in the specification. For example, in OCL "self" is used to refer to the context element. Constraints are often expressed as a text string in some language. If a formal language such as OCL is used, then tools may be able to verify some aspects of the constraints. In general there are many possible kinds of owners for a Constraint. The only restriction is that the owning element must have access to the constrainedElements. The owner of the Constraint will determine when the constraint specification is evaluated. For example, this allows an Operation to specify if a Constraint represents a precondition or a postcondition. An element import identifies an element in another package, and allows the element to be referenced using its name without a qualifier. Description An element import is defined as a directed relationship between an importing namespace and a packageable element. The name of the packageable element or its alias is to be added to the namespace of the importing namespace. It is also possible to control whether the imported element can be further imported. Semantics An element import adds the name of a packageable element from a package to the importing namespace. It works by reference, which means that it is not possible to add features to the element import itself, but it is possible to modify the referenced element in the namespace from which it was imported. An element import is used to selectively import individual elements without relying on a package import. In case of a nameclash with an outer name (an element that is defined in an enclosing namespace is available using its unqualified name in enclosed namespaces) in the importing namespace, the outer name is hidden by an element import, and the unqualified name refers to the imported element. The outer name can be accessed using its qualified name. If more than one element with the same name would be imported to a namespace as a consequence of element imports or package imports, the elements are not added to the importing namespace and the names of those elements must be qualified in order to be used in that namespace. If the name of an imported element is the same as the name of an element owned by the importing namespace, that element is not added to the importing namespace and the name of that element must be qualified in order to be used. If the name of an imported element is the same as the name of an element owned by the importing namespace, the name of the imported element must be qualified in order to be used and is not added to the importing namespace. An imported element can be further imported by other namespaces using either element or package imports. The visibility of the ElementImport may be either the same or more restricted than that of the imported element. An element is a constituent of a model. As such, it has the capability of owning other elements. Description Element has a derived composition association to itself to support the general capability for elements to own other elements. A packageable element indicates a named element that may be owned directly by a package. A package import is a relationship that allows the use of unqualified names to refer to package members from other namespaces. Description A package import is defined as a directed relationship that identifies a package whose members are to be imported by a namespace. Semantics A package import is a relationship between an importing namespace and a package, indicating that the importing namespace adds the names of the members of the package to its own namespace. Conceptually, a package import is equivalent to having an element import to each individual member of the imported namespace, unless there is already a separately-defined element import. MultiplicityElement is specialized to support the use of value specifications to define each bound of the multiplicity. An opaque expression is an uninterpreted textual statement that denotes a (possibly empty) set of values when evaluated in a context. Description An opaque expression contains language-specific text strings used to describe a value or values, and an optional specification of the languages. One predefined language for specifying expressions is OCL. Natural language or programming languages may also be used. An expression is a structured tree of symbols that denotes a (possibly empty) set of values when evaluated in a context. An expression represents a node in an expression tree, which may be non-terminal or terminal. It defines a symbol, and has a possibly empty sequence of operands that are value specifications. An element is a constituent of a model. Description Element is an abstract metaclass with no superclass. It is used as the common superclass for all metaclasses in the infrastructure library. A generalization between two types means each instance of the specific type is also an instance of the general type. Any specification applying to instances of the general type also apply to instances of the specific type. A value specification is the specification of a (possibly empty) set of instances, including both objects and data values. Description ValueSpecification is an abstract metaclass used to identify a value or values in a model. It may reference an instance or it may be an expression denoting an instance or instances when evaluated. A Type is a NamedElement that groups individuals according to some commonality among them, which might be characteristics they can have or constraints they obey. Types can cover any kind of entity, physical or computational, static or dynamic. For example, the type Person groups individual people, like Mary and John. The type declares commonalities among people, for example, they can have names and gender, or obey constraints, such as being genetically related to exactly two other people. A multiplicity is a definition of an inclusive interval of non-negative integers beginning with a lower bound and ending with a (possibly infinite) upper bound. A multiplicity element embeds this information to specify the allowable cardinalities for an instantiation of this element. Description A MultiplicityElement is an abstract metaclass which includes optional attributes for defining the bounds of a multiplicity. A MultiplicityElement also includes specifications of whether the values in an instantiation of this element must be unique or ordered. Semantics A multiplicity defines a set of integers that define valid cardinalities. Specifically, cardinality C is valid for multiplicity M if M.includesCardinality(C). A multiplicity is specified as an interval of integers starting with the lower bound and ending with the (possibly infinite) upper bound. If a MultiplicityElement specifies a multivalued multiplicity, then an instantiation of this element has a set of values. The multiplicity is a constraint on the number of values that may validly occur in that set. If the MultiplicityElement is specified as ordered (i.e., isOrdered is true), then the set of values in an instantiation of this element is ordered. This ordering implies that there is a mapping from positive integers to the elements of the set of values. If a MultiplicityElement is not multivalued, then the value for isOrdered has no semantic effect. If the MultiplicityElement is specified as unordered (i.e., isOrdered is false), then no assumptions can be made about the order of the values in an instantiation of this element. If the MultiplicityElement is specified as unique (i.e., isUnique is true), then the set of values in an instantiation of this element must be unique. If a MultiplicityElement is not multivalued, then the value for isUnique has no semantic effect. An element can own comments. The comments for an Element add no semantics but may represent information useful to the reader of the model. A comment is a textual annotation that can be attached to a set of elements. A comment gives the ability to attach various remarks to elements. A comment carries no semantic force, but may contain information that is useful to a modeler. A comment may be owned by any element. A Comment adds no semantics to the annotated elements, but may represent information useful to the reader of the model. A package is a container for types and other packages. Packages provide a way of grouping types and packages together, which can be useful for understanding and managing a model. A package cannot contain itself. A named element represents elements with names. Elements with names are instances of NamedElement. The name for a named element is optional. If specified, then any valid string, including the empty string, may be used. A typed element is a kind of named element that represents elements with types. Elements with types are instances of TypedElement. A typed element may optionally have no type. The type of a typed element constrains the set of values that the typed element may refer to. A parameter is a specification of an argument used to pass information into or out of an invocation of a behavioral feature. Semantics A parameter specifies arguments that are passed into or out of an invocation of a behavioral element like an operation. A parameter's type restricts what values can be passed. A parameter may be given a name, which then identifies the parameter uniquely within the parameters of the same behavioral feature. If it is unnamed, it is distinguished only by its position in the ordered list of parameters. A property is a structural feature of a classifier that characterizes instances of the classifier. Description Property represents a declared state of one or more instances in terms of a named relationship to a value or values. When a property is an attribute of a classifier, the value or values are related to the instance of the classifier by being held in slots of the instance. Property is indirectly a subclass of TypedElement. The range of valid values represented by the property can be controlled by setting the property's type. An enumeration defines a set of literals that can be used as its values. An enumeration defines a finite ordered set of values, such as {red, green, blue}. The values denoted by typed elements whose type is an enumeration must be taken from this set. DataType is an abstract class that acts as a common superclass for different kinds of data types. DataType is the abstract class that represents the general notion of being a data type (i.e., a type whose instances are identified only by their value). An enumeration literal is a value of an enumeration. A primitive type is a data type implemented by the underlying infrastructure and made available for modeling. A classifier is a type and can own generalizations, thereby making it possible to define generalization relationships to other classifiers. Semantics A Classifier may participate in generalization relationships with other Classifiers. An instance of a specific Classifier is also an (indirect) instance of the general Classifier. The specific semantics of how generalization affects each concrete subtype of Classifier varies. A Classifier defines a type. Type conformance between generalizable Classifiers is defined so that a Classifier conforms to itself and to all of its ancestors in the generalization hierarchy. Relationship is an abstract concept that specifies some kind of relationship between elements. A directed relationship represents a relationship between a collection of source model elements and a collection of target model elements. A redefinable element is an element that, when defined in the context of a classifier, can be redefined more specifically or differently in the context of another classifier that specializes (directly or indirectly) the context classifier. Description A redefinable element is a named element that can be redefined in the context of a generalization. RedefinableElement is an abstract metaclass. Semantics A RedefinableElement represents the general ability to be redefined in the context of a generalization relationship. The detailed semantics of redefinition varies for each specialization of RedefinableElement. A redefinable element is a specification concerning instances of a classifier that is one of the element’s redefinition contexts. For a classifier that specializes that more general classifier (directly or indirectly), another element can redefine the element from the general classifier in order to augment, constrain, or override the specification as it applies more specifically to instances of the specializing classifier. A redefining element must be consistent with the element it redefines, but it can add specific constraints or other details that are particular to instances of the specializing redefinition context that do not contradict invariant constraints in the general context. A redefinable element may be redefined multiple times. Furthermore, one redefining element may redefine multiple inherited redefinable elements. Semantic Variation Points There are various degrees of compatibility between the redefined element and the redefining element, such as name compatibility (the redefining element has the same name as the redefined element), structural compatibility (the client visible properties of the redefined element are also properties of the redefining element), or behavioral compatibility (the redefining element is substitutable for the redefined element). Any kind of compatibility involves a constraint on redefinitions. The particular constraint chosen is a semantic variation point. An instance value is a value specification that identifies an instance. A ImportableElement indicates a named element that is imported by a Namespace.