I understand that the individual use cases within a Use Case Diagram represent functionality. However, the lines going to and from the indiviual use cases do not represent data flows.
At the same time, for black-box testing (i.e., functional testing), when developing test cases, we need to specify data inputs and expected data outputs (amoung other things). So from where do we get insight about data flows, especially for larger scale efforts where data flows can get complex?
Not sure if this will answer your question but we have a one to many mapping from use cases to scenarios. The scenarios handle possible inputs and outputs. From there each scenario generally maps to a test case.