What is the different between Include and Extend Relationship in Use Case Diagrams?
The include relationship adds additional functionality not specified in the base use case. The <<Include>> relationship is used to include common behavior from an included use case into a base use case in order to support the reuse of common behavior.
The <<extend>> relationship is used to include optional behavior from an extending use case in an extended use case.
Note That:
If a base use case A includes use case B: then use case B must be done inside use case A.
If B extends a base use case A: then use case B can be done inside A (A can use B, but not always)
References
Use Case Driven Approach for Agile Development
