Testigophoenix, welcome to FlyerTalk!
The abbreviation *A most often refers to the airline members of the Star Alliance, one of three major alliances of airlines. Among the features of these three alliances is the ability to book award travel on any of them, even if your miles have been earned in only one member of that alliance. Also, the alliances may allow you to earn miles in your choice of program even if you fly another alliance airline.

As far as booking codes for upgrades, I wish the ranking correlated with the alphabet, but it ranges all over the alphabet. One airline's hierarchy for coach booking codes is, from highest to lowest rank: Y, B, E, M, U, H, Q, V, W, T, S, K, L, G.

Airlines may have some coach fares, well below full-fare, which offer an instant upgrade to first or business class fares. These are so called, H-up, Q-up, etc.