Control Sets are used for activating different schedules services, such as ringing services, restriction services, etc. Different lines can have different control sets to activate different services.
Lets say lines 61-64 are for department A. Set 221 is the control set for those lines. If you press Feature 871 on DN 221, lines 61-64 will go into "Night Ring" mode, or any other service you have defined. Lines 65-68 may have a different control set, or no control set, and therefore aren't affected by that service.
Hope this helps - this is one of the "Nortel-isms" that's kind of hard to explain.