A Planned test unit is a copy of a global test unit which is assigned to the template of a test plan.
By creating a planned test unit from a global test unit, you specify that the test unit has to be tested for the assigned test plan (i.e. the test cases contained within the test unit).
Creating a copy of the original test unit (as opposed to just a linked reference to it), you ensure that future changes to the global test unit do not modify the test plan with the planned test unit. This avoids erroneous falsifications of tests that have already been performed.