A very restrictive system might offer students a short list of possible combinations and ask them to pick one. An incredibly lax system might allow students to pick any combination they like. Both of these are easy to implement but any real university will have something in between and in this one way, at least, we’ll try to be realistic. The usual way to specify a set of combinations is with rules, like “If you take Statistics you must also take Probability” or “You must take one and only one of these three modules”. You find rules like these in a course handbook and the system’s job is turn those rules and turn them into an algorithm which approves or rejects a selection.