They are also different for different airlines as well. And not all codes on display are sellable for that one country either.
First class on some other airlines are usually represented by
A and F
Some other business class codes seen are C and I
Some other economy codes can be are N as well.
In the case for a Qantas domestic flight the codes are as follows and their meanings.
J - full published fae level Business Class
D - International flight connections for Domestic business class
Y - Full Economy Class.
U- Class not shown on availability screen but denotes upgrade from Economy to Business.
H, B, - Fully Flexible and full refundable levels of fare economy class
K - Fully Flexible but non refundable economy fare
M,L,V - Flexi fare saver type fares, flexible but at an extra fee
S and Q - wholesale holiday fares, must be booked with acomodation
E - Industry classed fare
Q - special unique fare for specials eg around Xmas etc esp on QF
O, N - cheapest class of economy fare - similar restrictions to M,L,V
Of course if the flight as international some classes do have different meanings.
So judging by that how full would you say that example of a flight is? 75%
Having said that only the airlines can really judge that, as seeing a H9
can really mean 9 'H' class seats on 42 or 57 of them. Generally though when you do start seeing quote a few "c" next to the class it mean that waitlists are closed for that booking class, or when there are a lot of "0" next to the booking class the flight are quite full indeed.
However inm terms of an international flight it is not uncommon for airline about a couple of months out to see about 120% of their seats, As in due course it goes down to the 100% booked or there abouts. Airlines have a complicated yield management program to work on such levels. It is only on the day when they are overfull that "bumping" can occur.