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A Test Set does not provide any direct information related with test results.
A Test Plan, on the other hand, tracks the results of some Tests in some version/sprint of the SUT. Thus, its purpose is to identify the Tests you want to run, to schedule executions for them and then group the respective results.
Thus, a Test Plan can be seen as a higher-level entity above Test Executions. It groups multiple Test Executions and presents a consolidated overview (i.e., the latest status) for the Tests tracked within it.
For a given version, or a given sprint, you may create one or more Test Plans to track the test results related to different tests (e.g., you may create a Test Plan for regression testing and another one for non-regression testing).
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