Y - Full coach fare
B, M - slightly discounted coach fare
H,Q,K - discounted coach fare
L,U,T - deeply discounted coach fare
F - Full First class fare
A - discounted first class fare
J - Full Biz class fare
D, I - discounted biz class fare
V - Upgrade bucket for domestic upgrades
X - Upgrade bucket for domestic upgrades on flights considered business instead of first
Z - Upgrade bucket for international upgrades (miles, PMUs)
N - Coach skymiles bucket
O - Business skymiles bucket
R - First skymiles bucket
E - Special coach bucket (Was used in the past for the fly 3 get 1 flight free promotion)
S - used to be student fares
That's all I know - there are others.