Your student number is the one you use in your exams to identify yourself. Your UAC number is whatever your student number is, but with a 1 in front of it. When you go to first apply through UAC, it asks you for your STUDENT number and the UAC PIN you will have recieved in the mail. When you complete the application, it will tell you your UAC number, but it is pretty simple to work out. You will need to use your UAC number for changing preferences, and anything else you do after you finish your application.