A travel agent
/Computer Reservations System
concept that refers to how many seats in a given class of service (e.g. First Class, Business Class, Unrestricted Coach, Bargain Coach)
* that an airline
wants to advertise as being "available for booking".
Contrast this with the normal concept of availability, which would refer to how many seats have not yet been filled. Instead, availability is an invented number, made up by each airline's yield management department or program...and the number is intended to strike the balance between making the most money per seat as possible vs. making sure that each plane is filled up (i.e. not having *any* empty seats, because an empty seat means $0 for that seat).
Travel agents can get availability for a flight at their travel agent terminal (ala Sabre, Galileo, Worldspan, Amadeus, etc.). Here is an example of what a travel agent might see when looking for what's available today going from Boston (BOS) to Los Angeles (LAX):
22JUL-SU-207A BOSLAX(BOSQLA) ET PT AC1 END
LAX ALTERNATE BUR LGB ONT SNA
1#AA 289 F7 Y7 B7 H7 M7 V7 Q7 K7 BOSLAX- 930A1240P 7 757 BB0
2#AA 181 F7 Y7 B7 H7 M7 V7 Q7 K7 BOSLAX-1100A 203P 8 757 LL0
3#AA 163 F6 Y7 B7 H7 M7 V7 Q7 K7 BOSLAX- 330P 640P 7 757 DD0
4#AA 223 P2 J4 Y7 B7 H7 M7 V7 Q7 BOSLAX- 530P 845P 6 762 DD0
Note that after the somewhat obvious stuff (carrier, flight number) there are letters and numbers: the letters are booking codes, and the numbers are the availability for each. For example, "Y" is the booking code that stands for "Unrestricted Coach", the class of service that means you'll pay the most for coach, you can get a refund, switch the ticket to another flight, etc. etc. If you book in "Y", you'll probably have a little label somewhere on your ticket that says Premium. This can sometimes be a code for the flight attendant that "Hey, although this traveller is not in first class, they did pay a lot for this ticket, they are probably a business traveller, and we want them to be happy and fly with us a lot, because we're making mucho dinero on them...
But back to the point: if you'll notice, all most all of these are "7". This is not because there are 7 seats left, it is because for this type of display, 7 is the maximum number (think: ancient computer systems, mainframes), and this means they would like to sell lots of this class of service for this flight. On the other hand, the "P" booking code may also be in coach (where there are probably more than 7 seats unsold), but the airline doesn't want to sell a bunch of these (cheaper) tickets.
*There is not a one-to-one correspondence between the booking code (e.g. "Y", "P", "F", etc.) and the class of service. There is a closer (but still not exact) relationship to something called a fare basis code--often the booking code will be the first letter of the fare basis code.