Here are all the current codes listed on CO.com when you click on fare class by a paid flight. Feel free to fill in the blanks if you can!
F, A: Paid First.
FN: First/Business - EasyPass Award for Elites and OP/MP cardholders
C, D, Z: Paid business
ZN: First/Business - EasyPass Award
R: ?
O: 3-cabin First Class Saver Award?
I: First/Business - SaverPass Award and Alliance Upgrades
Y, B, M, E, U, H, Q, V, W, S, T, L, K, G, N: Paid coach
YN: Coach - EasyPass Award for Elites and OP/MP cardholders
HN: Coach - EasyPass Award
XN: Coach - SaverPass Award for Elites
X: Coach - SaverPass Award
IN: First/Business - SaverPass Award for Plat Elites
JN: First/Business - Upgrade from Y Class for Elites
ON: 3-cabin First Class SaverPass Award for Plat Elites.
PN: First/Business - Upgrade from B Class for Elite and from M Class for Plat Elites
RN: First/Business - Upgrade using Miles/SWU/Vouchers/EUA