I know that American uses "J" as its full fare Business Class and "C" was its fare class for upgrades. My understandign is that this has now changed to "R." On all Oneworld carriers, "U" is a miles purchased Business Class ticket and "Z" is a miles purchased First Class ticket. Also, all Oneworld carriers use "D" as discount Business Class for RTW tcikets and "A" as the fare bucket for RTW First Class. Adding to the confusion, American also uses "A" for mile and certificate upgrades to First Class and "X" for electronic upgrades. While "F" is American's usual code for full-fare first class, it uses "P" for full fare first class on premium transcon (three-class) flights from JFK to SFO and LAX.

Basically, I do not think there is much uniformity in the designation of these fare codes across airlines.