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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

Use Case Analysis: How to Identify Actors?

USE CASE DIAGRAM – A NOTATION GUIDE

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