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UML Element Description
Actor
Actor    
An actor represents a set of roles that a user plays with an entity (system, subsystem or a class). Actors can be human or other systems, subsystems or classes that represent things outside of the entity. The actor "communicates" with the entity by sending a series of messages backwards and forwards.
Use Case
Use Case    
A use case represents the behavior of the entity, describing the interaction between the actors and the use case in a series of actions, with possible variants. It describes the value the entity provides to the actor with which it communicates. The use case never describes "how" the value is achieved, just "what" the value is.
Actor-Use case Association
Actor-Use case Association    
An actor-use case association is a semantic relationship between an actor and a use case. This association is typically not named and consists of exactly two association ends. The association ends are simply the end parts of the association where they connect to the actor at one end and the use case at the other. It is the association's ends that may have a number of adornments that express the rich structural semantics of the relationship.
Subject (System) Boundary
Subject (System) Boundary    
A subject (system) boundary is a type of partition that represents the boundary between the thing you are representing with the use cases (inside the boundary) and the actors (outside the boundary). Its most typical usage is the boundary of an entire system. Use cases can be used to represent subsystems and classes and so the boundary may be more specific than an entire system. A package with a stereotype topLevel can be used as a boundary and name space within the use case model to denote the same thing as the use case boundary.
Include
Include    
Include is a relationship that may exist between two use cases, a base use case and an addition use case. It means a base use case may insert behavior from an addition use case. The relationship is said to be unconditional in the sense that if the insertion occurs; all the behavior of the addition use case is unconditionally inserted.
Extend
Extend    
Extend is a relationship that may exist between two use cases, the base use case and the extension use case. It is said to be conditional in the sense that a set of extension points defines conditions under which the insertion will occur. The extension points are defined in a separate section of the use case.
Extension Point
Extension Point    
An extension point is a feature of a use case and acts as a reference to a location in the (extended) use case at which the behavior of the use case may be extended by the insertion of additional behavior from another (extending) use case. A use case may have any number of extension points and these are used in conjunction with the extend relationship which contains reference to the extension points and conditions which govern whether the extending behavior will be inserted.
Actor Generalization
Actor Generalization    
Actor generalization is a way of factoring out commonality that exists between a number of actors. The actors must be of the same type or family, and have associations with the same set of use cases. The specialized actors may add or remove associations, and can always stand in the place of the parent or more generalized actor. The more generalized actor is often, but not always, abstract.
Actor Generalization (tree)
Actor Generalization (tree)    
Actor generalization can be represented in a single path (tree). When a number of generalization relationships exist and the children have the same parent the lines can be combined into a single path and drawn with a tree. This doesn't change the semantics of the individual relationships; it is simply a more compact and convenient style for representing the relationships. A tree that has three branches represents three discrete and separately defined generalization relationships.
Use Case Generalization
Use Case Generalization    
Use case generalization is a relationship between two use cases indicating that the child (tail end) is more specific than the parent (arrow head end).
Use Case Generalization (tree)
Use Case Generalization (tree)    
Use case generalization can be represented in a single path (tree). When a number of generalization relationships exist and the children have the same parent the lines can be combined into a single path and drawn with a tree. This doesn't change the semantics of the individual relationships; it is simply a more compact and convenient style for representing the relationships. A tree that has three branches represents three discrete and separately defined generalization relationships.
Comment (Note)
Comment (Note)    
A comment can contain textual information or graphical symbols that add additional information to a diagram or model. The information can take many forms from very formal, in the case of constraints, to conversational, in the case of working notes. Comments can be placed anywhere in a model or diagram, and simply by their proximity and alignment; they can be applicable to one or more elements. They may also be attached either to one or more elements by a comment attaching line.
Comment (Note) Attaching Line
Comment (Note) Attaching Line    
A comment attaching line is a dashed line that is used to attach a comment (note) to a model element in a diagram. The comment (note) attaching line is not a UML relationship and has no semantics beyond signifying that the contents of a comment apply to the model element it is attached to. A comment may be attached to any number of model elements or may exist in isolation and by placement in proximity to one or more model elements signify that it applies to those elements.
Actor
Actor    
An actor can be represented by a classifier (rectangular) symbol. Both this form and the more commonly seen stick-person form have the same semantics. The rectangular form is sometimes used when representing a system actor but it can also be used to represent a human actor. When an actor has properties and these need to be displayed as part of a diagram the rectangular form can be used; the properties appear in a separate compartment.
Actor Instance
Actor Instance    
An actor instance is a specific instance of an actor using the entity being modeled, which is most typically a system but could be a subsystem or a class. It has unique identity as well as attribute values, which determine its state at any point in time. An actor instance communicates with one or more instances of use cases sending message instances.
Use Case Instance (Scenario)
Use Case Instance (Scenario)    
An use case instance is a specific instance of a use case. It represents the behavior of the subject, describing a particular instance of the interaction between the actors and the use case. A use case instance communicates with one or more instances of actors sending message instances to achieve the value represented by the use case instance. The use case instance never describes "how" the value is achieved, just "what" the value is.
Link
Link    
A link is an instance of an association. While an association connects classifiers to show structural relationships, the link (association instance) connects classifier instances. It is a list of the references between classifier instances. Its most common expression is a reference to two classifier instances in its binary form: an n-ary association will have n-ary links. Properties such as aggregation can be used with a link, however multiplicity and ordering may not as they have no meaning.
Association End
Association End    
An association end is the endpoint of an association that marks the point at which the line connects with a classifier. An association end has a name (role name) and a series of properties that determine the semantics of how the classifier participates in the relationship. A binary association has exactly two association ends and an n-ary association has 'n' association ends.
IsNavigable
IsNavigable    
The value of the isNavigable property specifies whether an association can be traversed from the source (far end) to the target (near end). It is indicated graphically by an open headed arrow.
Multiplicity
Multiplicity    
multiplicity is a property of an association end that specifies the number of instances of the classifier connected at the target end (near end) that can be associated with a single instance of the classifier at the source end (far end). It is specified in the form of a string stipulating a possible infinite number governed by a lower and upper bound.
Package Visibility
Package Visibility    
When the value of package is specified for the visibility property of an association end it signifies that any classifier that is contained in the same package as the association (or any nested sub package) may traverse the relationship and use the role name in expressions.
Private Visibility
Private Visibility    
When the value of private is specified for the visibility property of an association end it signifies only the classifier connected at the source (far) end may traverse the relationship and use the role name in expressions.
Protected Visibility
Protected Visibility    
When the value of protected is specified for the visibility property of an association end it signifies that the classifier at the source (far) end of the association and its descendants may traverse the relationship and use the role name in expressions.
Public Visibility
Public Visibility    
When the value of public is specified for the visibility property of an association end it signifies that other classifiers in addition to the classifier connected at the source end of the association can traverse the relationship and use the role name in expressions.
Unordered
Unordered    
The value unordered for the ordering property signifies that instances of the classifiers at the target end of the association are not ordered. This is the default value for this property and if a value for the property is not specified then the value by default is unordered. The multiplicity at the target end must be greater than one since a value of one or less would specify a set with one or less elements, for which ordering would be meaningless.
Ordered
Ordered    
The value ordered can be set for the ordering property of an association end. This signifies that instances of the classifiers at the target end of the association are ordered. Any operation that adds or changes the instances is responsible for maintaining the order. The multiplicity at the target end must be greater than one since a value of one or less would specify a set with one or less elements, for which ordering would be meaningless.
Element Import
Element Import    
An element import is a directed relationship between a packageable element (arrowhead end) and an importing namespace (tail end). The element that is pointed to is added to the namespace, allowing the element to be used in the namespace without any qualification. A name can be added to the element import relationship, which is used as an alias (alternative name) for the element in the importing namespace. The visibility of the element in the importing namespace is by default public.
Element Import (Access)
Element Import (Access)    
An element import with the keyword access is a directed relationship between a packageable element (arrowhead end) and an importing namespace (tail end). The element that is pointed to is made accessible to the namespace, allowing the element to be used in the namespace without any qualification. It is different from an element import with keyword import because it does not import the element into the namespace but rather just allows it to be accessed. A name can be added to the element import relationship, which is used as an alias (alternative name) for the element in the importing namespace. The visibility of the element in the importing namespace is by default public.
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