Just to add to what Manflyer and StarG said
when it shows F9 A9 C9 D9 J9 Y9 M9 and all that, it means
that the airlines is willing to sell you at least 9 seats in those classes.
Note that the numbers can't be added up - F9 A9 doesn't necessarily mean you can book 18 first class seats. It doesn't work that way.
As a general rule of thumb from observation at our company travel agent (who have terminals in my office)- most airlines show different "max" numbers when the flight is wide open i.e fairly light.
SQ for example shows a "max number" of 4 for first and business, 9 for economy.
UA shows a max of 9 all classes
TG F - 2, C-2, Y-9
MH F -4, C- 4, Y-9
NZ C-4 (IIRC), Y-7
OS- C-9, Y-9
Just some observations can't think of more off the top of my head and i think this could be different from system to system. My TA uses Abacus.