Just to add to that: You probably know that particular fares need to be booked in particular classes - usually the class that is the first letter of the fare basis code. The display shows the minimum number of seats that the airline will sell in each particular booking class at the present time. 9 is the maximum that any display can show, and many airlines use 9 although some limit it to 7 or 4.

All that you can really tell when everything shows 9 is that the airline will still take reservations for at least 9 seats in each cabin. The numbers are not cumulative - Y1 B1 H1 may indicate that the airline is only prepared to take one more economy class booking but doesn't care whether you take it for a Y class, B class or H class fare. If you were to book one seat, you might find that the display then immediately goes to Y0 B0 H0.

This way the airline can manage the yield it's getting. In the lower classes, there might for example be something like M5 R5 V2 N2 - the airline will sell only two more seats in the deepest discount public classes of V and N, but will sell a few more in the more expensive M and R classes. After it's sold two more N class seats, the display may then go to M3 R3 V0 N0 - so the next person will have to buy the more expensive R class fare if he wants to travel on the flight.

You can't necessarily tell from these displays whether any cabin is already overbooked - this only gives an indication of how much more the airline is prepared to book. All 0's may be merely a full flight where the airline is not prepared to overbook, or it may be a flight which is already overbooked to the limit.

You'll sometimes also some special symbols instead of the numerical digits. Some of the more common are these: L means waitlist only, R means on request only, C means class closed (no more waitlists/requests will be accepted). A means available and is often used where an airline's connection to the display is more rudimentary and will not show numerical availability.

Because airlines will waitlist for some classes, a display of all zeros is actually not quite what it seems. The really really full/overbooked flights will show YC BC HC ... where nothing more will be accepted for love or money, at least from Joe Public. I've seen operating flights where every single class was closed (although that can also sometimes mean that the flight is about to be cancelled).

However, despite these complications, you can get a feel for what the flight is looking like at any particular time. All 9s is a flight that is wide, wide open - maybe only a handful of seats have been sold. All or most classes showing 0 is likely to be a flight which is well booked and where the overbooking levels are being actively managed to try to ensure that the flight clears by departure time with no oversell - which most do.

One more word of warning: The airline may show different displays to different countries. For example, a UK agent may see Y5 B3 H1 when a US agent sees Y0 B0 H0 - or vice versa. That's another tool that the airline uses in yield management.