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 sprint. Tests sets are created by default within the same project as the regression plan. You can move test sets to another (e.g., lower level) project if appropriate for work tracking purposes.