I work in the travel business and have found what seems to be almost a pattern for class designation:

First: F, P (full fare) / A (discounted first)

Business: J, C (full fare) / I, D, Z (discounted business)

Economy: Y (full fare, usually used for OW) / B, M, H, K (normal fare) / Q, V, T, L (discounted economy).

Full fare means usually no minimum stay, no advance purchase and no penalties, while the discounted classes are usually inclusive of these restrictions. Also, the discounted classes in all types of services are usually the ones used by consolidators, for upgrades, secured (not viewable to the public), etc.

Finally - I think that the airlines control seats by using different classes for different points of origin and destinations around the globe. The lowest fare from the U.S may be in L class while the lowest fare TO the U.S will be in V class.