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.

Explanation

An extension point labels a place in a use case where the behavior can be augmented or altered by another use case. An extension point works in conjunction with the extend relationship which contains a list of conditions under which the extension will occur. The significance of the extension point is that it is listed in a separate compartment of the use case allowing it to be managed independently of the body of the use case description. Effectively this allows modelers to simplify the use case and not clutter it with unusual or exceptional behavior. The extension point refers to points in the use case but the body of the use case does not refer to the extension points. We see examples of this in our daily lives when we are checking in at an airport and our bags exceed the limits allowed we would typically be directed to an excess baggage counter. The use case could be called 'Perform Check-in' and the extension point might be called 'Measure Baggage' which would refer to a point in the body of the use case where the baggage is weighed or sized. The extend relationship would define a condition 'Exceeds Limit' and the extending use case would contain a description of the behavior to be inserted when the point is reached and the condition satisfied. (Notice the use of words such as 'measure', which abstract the notion of weighing or taking the linear dimensions, either or both of which are used in different regions of the world).

Related Entries

Use Case  Owned Element An use case is related to a extension point because the use case owns the extension point. It appears in a separate compartment and defines a point in the use case where the behavior of the use case is extended. The extending behavior is defined in another use case which is related to the extended use case by an extend relationship.

Extend  Used Together An extend is related to a extension point because it is the relationship that relates the extending use case (additional behavior) to the extended use case (basic behavior). An extension point appears in a separate compartment and defines a point in the extended use case where the behavior of the use case is augmented. The additional behavior is defined in another use case which is related to the extended use case by an extend relationship