Only those subjects with the subject group type as 'PS' or 'University Wide' can be registered as additional subjects. You can check the subject details via eStudent > Information > Subject Search. The subject group type can be found in the 'Subject Group Details' block.
Students of Full-time Undergraduate programmes can register the additional subjects outside curriculum to broaden their perspective during the last two days of the Add/Drop Period via eStudent, subject to not exceeding the maximum credits allowed and the availability of vacancies.