The workflow is represented by an activity diagram, which depicts conditions, constraints, and sequential and concurrent activities. On the other hand, A Use Case is intended to display only the functionality of the system, not how it is done. In other words, an activity diagram shows 'How,' whereas a use case shows 'What' for a specific system. Both of them have different levels of abstraction. An activity diagram can depict a business process (high-level implementation) to a stand-alone algorithm (ground level implementation). On the other hand, use cases have a low level of abstraction.
