To solve this question, the user needs to understand the purpose of test completion criteria in a test plan.

Test completion criteria are used to determine when to stop testing. The criteria ensure that the testing process is complete and that all necessary tests have been executed.

Option A: This is incorrect because test completion criteria are not used to determine when a specific test has finished its execution.

Option B: This is incorrect because test completion criteria are not used to ensure that the test case specification is complete.

Option C: This is incorrect because test completion criteria are not used to set the criteria used in generating test inputs.

Option D: This is incorrect because test completion criteria are not used to determine when test planning is complete.

Option E: This is correct because test completion criteria are used to plan when to stop testing. Once the criteria have been met, testing can be concluded.

Therefore, the answer is: E.