Are you talking about the established process or actual practice? If actual:

Pre-board (50% of aircraft)

First class (All SP and most FO's board now too)

Sky (remaining FO's and all credit card holders board)

Zone 1 (this is just for giggles, everyone in zone 1 is already on the airplane)

Zone 2-3 - (the remaining 5-10 people)

This is all subject to change without notice based on full moons, agent attitudes, text volume of agent, number of DYWIA's on flight, etc.