Why are there “reserve capacity” in some courses?

Course departments may request to reserve certain or all places of a course for students in particular programs during the enrollment period. Students who have officially declared their major or minor (where applicable) in the specified programs can occupy the reserve capacity.

For UG courses, most reserve capacity would be released as free seats when add/drop period starts if they are not taken back by the course departments. Reserve capacity in PG courses would not be released as free seats unless the instructor / course department intends to do so.