A Template (also called Test template) defines which test units (planned test units) and test cases (planned test cases) are actually being tested within a test plan.
Usually, you do not test all test units and test cases within one single test plan but instead group them logically into several different test plans. Each test plan therefore contains its own template which defines the actual test units and test cases to test.
You can add test units and test cases to a test plan as long as there are no test runs within the test plan.
The nesting hierarchy of the global test units and test cases can be the same as within a template, but need not necessarily be: i.e. you can change the nesting hierarchy of test units and test cases within a template to be completely different from the original nesting hierarchy within the global test units and test cases section.
In addition, you can add the same global test unit or global test case multiple times to a single template.