Some courses require instructor permission in order to register; see the class attributes section in the DukeHub. In order to enroll in the course, the student must obtain a permission number from the instructor. Some instructors will set deadlines after which a permission number will no longer be valid. Students are encouraged to request a permission number only for those courses they really want to take; then go ahead and register as soon as possible prior to the permission number’s expiration date. Permission numbers cannot be reused.