I think it's probably good to write use cases for the business processes or functions you're responsible for.

If you're responsible for a function that needs test data from another module, you don't need to think about the source of the data, you need to mock out all the possible data and verify that the function you're responsible for handles it correctly.

For example, if the previous module gave you A.B.C.D, your test case should start with your function, but the preconditions should say that you need to consider A.B.C.D data, you are responsible for the correctness of the function.