Often candidate use cases include items that require an actor's intervention to complete the activity. This intervention is itself a separate activity that could become its own use case. Consider, for example, a use case Order Books. When the customer completes an order, the system sends a message to the credit card company to approve the charge. It is unclearwhether the response from the credit card company is a separate use case or not. In addition, this informal description introduces vocabulary (“message”) that does not belong in the Bookstore domain.
This section provides guidance on working with use cases that span several domains and when a use case should be decomposed into other, smaller use cases.