Generally speaking, we tend to use them when we get lazy to indicate the class of service travelled as well.
Eg. J is Business Class on most airlines, Y is full economy on all? airlines. whY is Economy.
Here's some I can think off the top of my head that we use regularly. (but all mentioned in the linked thread)
First Class:
F- Unrestricted First
A- OW Discount First (used also in Oneworld explorers in First)
Business Class:
J- Unrestricted First
D- OW Discount Business (used also in Oneworld explorers in Business)
C- More so on Star Alliance but Business Class
Economy Class:
Y- Full Unrestricted Economy Class
O- Qantas Base Red-E-Deal (will accrue points to AA)
N & Q- Qantas Red-E-Deal (won't accrue to AA)
L- OW Economy (also used in Oneworld Explorers in Economy)
