I am trying to figure out how upgrades work overall.

I realize Exec Platinums move above the Platinums, but what I do not understand is how do they decide the order of the Exec. Platinums.

If I book on AA I get 4 choices.

1) Choice
2) Choice Essential
3) Choice Plus
4) First.

Obviously First is First Class. Are people that book Choice Plus moved above Choice Essentials and Choices if they are all Exec. Platinum. I know booking codes matter, but I just checked a flight and all were listed as V. I have no idea what booking codes mean unless they are different classes. For example I understand that P is higher than V, but I see all these for discounted coach: H, K, M, L, W, V, G, Q, N, O, S. Are some of these better than others and how do you choose the better ones? Forgive my ignorance, but I have yet to find someone who can explain how upgrades work so I thought I would ask here.