If a use case contains a segment of behavior of which only the result, not the method for getting the result, is of any importance to the rest of the use case, this behavior can be factored out to a new inclusion use case. The original use case then becomes the base use case in an include-relationship with the inclusion use case. See also Guidelines: Use-Case Model.
An include-relationship between two use cases means that a use-case instance following the description of the base use case also needs to follow the description of the inclusion use case in order to be complete.
The include-relationship can help clarify a use case by:
Generally, more than one use case must include an inclusion use case to make it worth it to maintain an extra use case and the include-relationship.
Only the base use case knows of the relationship between the two use cases; no inclusion use case knows what other use cases includes it.
Describe the include-relationship by briefly stating the purpose of the inclusion, as well as the location in the base use case at which the inclusion is to be inserted.
When describing the flow of events of the base use case, you should refer to the inclusion at the location in which the inclusion is inserted.
If a use case has segments of behavior that are optional or exceptional in character, and that do not add to the understanding of the primary purpose of the use case, factor those out to a new extension use case. The original use case then becomes a base use case, to which the extension use case has an extend-relationship. See also Guidelines: Use-Case Model.
In the base use case you declare extension points, which define where in the base use case extensions may be made. See also Guidelines: Use Case.
Complex sub-flows and optional behavior are the first candidates for being partitioned out into extension use cases. Often this behavior can be quite complex and hard to describe: including it in the flow of events of a use case can make the "normal" behavior harder to see. Extracting it should improve the comprehensibility of the use-case model.
Make sure that the flow of events of the base use case is still complete and understandable by itself, without any reference to the extension use case.
Only the extension use case knows of the relationship between the two use cases. The base use case only knows it has extension points, it doesn't know what extension use cases are using them.
Briefly describe every extend-relationship you define. Define the conditions that must be met for the extension to occur. Make sure to define the extension point in the base use case at which the extension should be inserted.
If two or more use cases have similarities in structure and behavior, you can factor out the common behavior to create a new parent use case. The original use cases will then be child use cases in generalization-relationships with the parent. The child use case inherits all behavior described for the parent use case. See also Guidelines: Use-Case Model.
A generalization-relationship between two use cases means that when a use-case instance follows the description of a child use case, it also needs to follow the description of the parent use case in order to be considered complete.
Generally, for it to be worth it to maintain a parent use case and a generalization-relationship with a child, there needs to be at least two child use cases inheriting from the same parent. An exception is if you have two use cases where one is a specialization of the other, but both need to be independently instantiable.
Only the child use case knows of the relationship between the two use cases; no parent use case knows what child use cases are specializing it.
To assist others in understanding the model, you should briefly describe the generalization-relationship. Explain why you created the generalization-relationship.
In the flow of events of the child use case you need to explain how the child will modify the inherited behavior sequences by inserting new segments of behavior.
Actors will have common characteristics that you should model by using actor-generalizations. This part of the work is best performed after you have made your first attempt at a use-case model.
Write a brief description of the actor-generalizations, and include them in use-case diagrams for further clarification.
You should continuously discuss the incorporation of include-, extend-, and generalization-relationships with the customer and the users, and see that they have a clear understanding of the resulting use cases and actors, and that they agree on their descriptions.
Check the use-case model at this stage to verify that your work is on track, but do not review the model in detail. You should review and discuss the newly incorporated use cases and relationships with the customer and users so that they have a clear understanding of the use cases and agree on their descriptions.
You should also consider the checkpoints for the use-case model while you are working on it. See especially checkpoints for actor, use case and use-case model in Activity: Review Requirements.