Hi Niranjan,
Using rotations determines how many times to run a test case using specific datasets which are determined on the 2nd tab in the execution set creation form. So if you make your selection for 200 datasets on the 2nd tab and you want to run the test case once using only these datasets, you can leave rotations to the default value of 1. That way, you will have 200 executions with each run using a specific dataset (the 400 will be ignored as you did not select them on the 2nd tab).
To answer your second question, that all depends on what you select on the 2nd tab. If some datasets are selected while others are not, only the selected ones will execute.
Regards,
Subject7 Team