A Test Case is a stereotyped Use Case element. You might use it to extend the facilities of the Testing window, by applying element properties and capabilities to the tests of a feature represented by another element or - more appropriately - set of elements. That is, you can define in one go, in the Testing window for the Test Case element, the details of the tests that apply to each of several elements, instead of recording the details separately in each element.
Within the Test Case element properties you can define test requirements and constraints, and associate the test with test files. You can also link the element to Document Artifacts or (in the Corporate, Unified and Ultimate Editions) directly to a Linked Document, such as a Test Plan.
The Test Case element enables you to give greater visibility to tests, in the Browser window, Diagram List, Package Browser, Model Search, Relationship Matrix, Traceability window and reports.
The Test Case element is available through the 'Use Case' and 'Maintenance' pages of the Diagram Toolbox.