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You can associate Test Executions with Test Plans by editing the custom field Test Plan in the Test Execution issue view page, or directly in the create issue screen in the Test Execution Details tab.
If the Test Execution is associated with a Test Plan and contains Tests that are not in the Test Plan, a warning message will be displayed telling you how many tests in the Test Execution are not in the Test Plan with a button to add these tests to the Test Plan.
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