A Test unit exists in order to group test cases. When operating with a large number of test cases, it is advisable to group test cases together, e.g. place all logically related test cases into one group.
Test units can be nested as and where needed and each test unit can contain an unlimited number of test cases. Test units are, if you will, similar to folders inside Windows Explorer which group files (for Zeta Test: test cases) together.
Test units are created "globally", i.e. all test units created are independent of any specific test plan. You assign test units and test cases when selecting them for a specific test plan.