During the normative study period, students are generally assigned study plans by Schools through Advice on Course Selection. If you intend to add extra courses on top of the study plan, you should submit an application via Application for Add/Drop during the add/drop period.
There could be several reasons for applying to add extra courses:
Applications will be considered case-by-case by Schools, with due consideration to factors such as availability of places and compatibility of class schedules. Results will be released within the Application for Add/Drop.