Let's say, I want to model an application which allows users to model class diagrams. The high level use case can be modelled as UC1:Model Class Diagram, which refines itself into UC11: Model Class, UC12: Model Connection, UC13: Model Composition, etc.
Since UC11, 12, 13 are part of UC 1, I used the include-Association. Unfortunately, the UML specification says that included use cases are essential parts and if you would leave one of them out the high level behavior could not be achieved any more.
But in this example a valid class diagram can be created without modelling a connection or a composition, so these use cases are optional.
To boil it down to an essence: How can optional use cases be modelled in UML while providing a mechanism for reuse (like the include association)?