In use case diagrams , the relationships between use cases may include "include" and "extend."
* Include indicates that a use case incorporates another use case as part of its behavior.
* Extend shows optional behavior that extends the base use case under certain conditions.
These relationships help modularize functionality and clarify dependencies.
BABOK Reference - Technique: Use Cases and Scenarios (Section 10.49):
"Use cases can have relationships such as include or extend to indicate reuse or conditional behavior extensions."