Originally Posted by PDX-RDM-AVL-TPA

I think I can list the paid fare classes in order from memory - high to low.
Business (generally international): J, D
First (domestic): F, A
Coach (upgradeable internationally): Y, B, M
Coach (upgradeable for non-elites or with miles or PMUs domestically): Y, B, M, H, Q, K
Coach (not upgradeable except for elites flying domestically using complimentary medallion upgrades - not with PMUs or miles even for elites) L, U, T.

Note that on some shorter (Latin America, Caribbean) international flights, complimentary medallion upgrades are possible like on domestic flights.

Complete list of normal paid fare classes - high to low:
J, F, D, A, Y, B, M, H, Q, K, L, U, T.