Its my understanding that you take the primary defender on a drive that originates from your area. Any help defense that may come at the hoop can be taken by other two officials depending on angles.
There is no hard and fast rule, as plays and angles develop, particularly at the bottom of the circle (that is not actually marked on an NCAA court
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And block/charge, just depends. Have to hold your whistle if you are L and it is the primary defender. Similarly, must hold your whistle if its help defense and you are C or T. One evaluator told me its fine if you blow it as well, but you better realize you have a double whistle and make eye-contact before giving prelim.