When entering Test Case Results from the Iteration Status Page:
If entering results via the gear icon, the Test Set is automatically set. The Test Set associated to the Test Case Result is inherited from the Test Set under which the results are being entered. This is easily recognizable from the Test Set hierarchy on Iteration Status.
If entering Test Case Results from within a Test Case itself, the Test Set will not automatically be populated. Test Cases can be associated to multiple Test Sets, so it’s necessary for the user to determine which Test Set the results belongs to. This requires a manual selection of Test Set.