Use Case Model

The use case model includes the use case diagram, as
well as narratives and scenarios for each use case.
The purpose of the use case model is to establish
what the users expect to see when they interact with
the system. The view is from outside of the system,
as though it is a black box. Scenarios provide test
cases for each use case and thus are critical resources
for the rest of the project. These test cases can and
should be applied at each iteration of the analysis and
design process.

The use case model describes what the users expect
to see when they interact with the finished system. These
expectations are expressed like goals, like withdraw money
and transfer funds. These goals imply functionality. For example,
the user must be able to provide the bank account number
and the amount of money to withdraw. The use cases provide
the basis for functional  partitioning.