Class orders determine what each class will do when the macro main loop releases to the class. Each class can go through 4 stages depending on how you have it set up for in and out of combat and what /cc modes you have set for it.
Base Order will be checked at the start of every class call. It is checked inside and outside of combat. Its purpose is specific to each class. Some classes check xtarget settings, some check if their pets are still fit for combat, Some check ammunition, and so on. Things that classes need that you don't want to waste time on while in active combat.
Pre order is checked at the onset of an engage, not to the point to /cc engage %, even if the mob is 100%, this is the first part checked. This list order will get continually checked until the mob reaches your engage %
Main Order is the actual combat routine. Once an engage has reached your engage point, this loop will be maintained until the mob is dead or you force a stop, or the assist changes target.
Once the Main Order has been completed, this will be executed 1 time before releasing the class code back to the main macro loop.