On the AADSAS site, there is a guide for application. It is a lengthy guide that has the school informations, GPA calculations and university codes and GPA calculations. There is also a very detailed guide about the classes that would count toward your GPA calculation and their codes. When you fill out your application you have to use this table to find the codes for your classes.
It basically goes hand in hand with common sense. All the classes that sound like science are counted in science GPA. Biologies, physics, math, chemistry, ....