This feature is available in the Ultimate Version only.
Test Sets are primary workitems (similar to story, backlog item or defect) that contain all the regression tests in a test suite. After you have added regression tests to a test suite, you can then create a test set that can then be scheduled to a release or a sprint. Tests sets are created by default within the same project as the regression plan. You can move test sets to another (for example, a lower level) project if appropriate for work tracking purposes.