Seats in each of the three travel classes on a plane are sold under different sub-classes for revenue management purposes. For CX, the the sub-classes are:

First Class: F and A
Business: J, C, D, I
Economy: Y, B, H, K, M, L, V, S, N, Q, O

(In a typical decending order of $ value, i.e. a F tix would cost more than an A tix)