6.6 Modelling the relationships between use cases
There are two situations when you would consider adding details to a use case diagram:
to identify a common task (and its associated scenarios) that is shared between two or more use cases;
to record any alternatives or additions to the main success scenario as separate use cases.
In both situations, the new tasks are shown as new use cases (ovals) and, as you will see below, the UML provides a suitable notation (known as a stereotype) to represent the relationship between the original use cases and the new ones.
The main disadvantage of this approach is the additional complexity they bring to a model in contrast to the simple use cases considered previously. The best advice for you as a requirements analyst is to remember why you are creating the model and who it is for.