For a domestic mileage-based upgrade, from coach to first class, on a two-class aircraft, you should be looking for "A" class.

Here's a brief summary of the fare codes on AA, as I understand them:

A = Upgrade from coach to first on a domestic 2-class flight, using miles or VIP.

C = Upgrade from coach to business, using miles or VIP.

R = Upgrade from coach to business, using e-upgrades (500-milers). This class will not show up until 100 hours before departure (not even as R0).

X = Upgrade from coach to first, using e-upgrades (500-milers). This class will not show up until 100 hours before departure (not even as X0).

T = Free coach ticket using miles.

U = Free business ticket using miles.

Z = Free first ticket using miles.


Hope that's helpful.