It varies by airline. These are the major fare codes for UA:

First: F, A
Business: C, D
Discounted, restricted business: Z
Economy: Y, B, M, E, U, H, Q, W, V, S, T, K, L (generally in that order, from full fare to the lowest discounted fares; there are probably some other, more obscure ones)

I think if you sort the seatcounter results by "fare class" it will put them roughly in order from lowest to highest.

Thanks ms_go.