My latest information (but could well be out of date)

R, Y - Economy Flex Plus
K, U, B, M - Ecconomy Flex
T, L, Q, N - Economy Saver
X, V - Economy Special

J, C - Business Flex Plus
I - Business Flex
O - Business Saver
H - Business Special

Premium Economy seems to be W and E. Flex Plus and ?

It's the first letter in the fare code on your e-ticket.