you can also see, for some code shared flights,
the availablity is different for the same physically airplane.

e.g. i once tried to RTW trip with BA, they can't take any seat via their system (booked under BA5xxx). then i told the agent to look under the operating airlines code (EIxxx), there are more than plenty of seats there.

Even if you see seats available "on the surface".... P9 F9 B9 Y9 V3 etc.
these are the number of seats still for sale. The flight may very well be oversold or overselling at any point. For example the coach cabin may hold 132 people and the flight may actually have sold 132 coach seats. Nevertheless there may be an additional 10, 20, 50 seats still showing "available" and will be sold in addition to this already "full" flight.
Good luck, you really can't get the true picture unless you have actual access to each airlines' system.

the airlines' own system shows the actually number of bookings vs the # seats avaliable.

then there a some 'human' (instead of computer) called Revenue/Yield management who look at them each day and determine how many seats to release to the system (GDS/CRS).

e.g. if there are 50J seats, 150Y seats. and the current booking is, say 40 J+D, 145 Y+B+M+etc

he would say, i would release 10 for J, 10 for Y/B, and 5 for M....if i oversold Y, i can upgrade them into J...etc

in addition, they have good historical data, which allow them to say...usually the no show is 5%, so i would sell 103 tickets for 100 seats, and be safe 90% of teh time. That is why sometimes they have to ask for volunteer. some airlines, like cx/sq are in general more conservative, they would sell 101 seats instead of 103.

Only the internal airline systems (this does not mean their web booking engines) will show true seat inventory. CRS and ITN will show seat inventory for sale that is updated 3-4 times a day. depending upon the airline/system, a number 8 or higher or 4 or higher means they have at least that number of seats for sale in that class. When the number drops below this number, that means it is the exact number of seats they have left.

CRS systems do not always have up to date information and may show inventory that is not there. The only way to see up to date inventory is to gain direct access to the airlines systems.

Seat inventory is very fluid. If you do not see the class of seat you need, check back later to see if yeild mangement has released more seats.

You can also find sold out seats by looking at connections using point to point searches.

Airlines will always oversell expensive coach tickets becasue there is always ample number of people willing to be bumped for cheaper costing travel vouchers.

D in business, means D o not even bother to try to upgrade with points or certs. It is D iscounted too much. I have such a ticket for a cruise, where I paid lots extra for business, but it is a D (D arn) ticket.

As always that depends, LH lets you upgrade
D to First without problems, as Rudi did many times. It depends like most of this on the airline and the programm.