Chapter 9. Include vs. Extend
It may seem confusing to have two kinds of relationships between use cases that both imply adding a subflow into another flow. However, as described in the two previous chapters, their purposes are very different. The include relationship is intended for reusing behavior modeled by another use case, whereas the extend relationship is intended for adding parts to existing use cases as well as for modeling optional system services.
When modeling behavior common to several use cases, it may at times seem difficult to decide which of the two relationships to choose. The answer to the following question often clarifies whether an include relationship or an extend relationship is to be used: Which of the two use cases is ...
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