They represent the minimum number seats available for sale at that moment in that bucket.

Within each cabin they are NOT additive so the F9 A7 doesn't necessarily mean there are 16 seats in the F cabin to sell. It means there are at least 9, of which 7 can be sold from the discounted A bucket.

The numbers also do not necessarily change linearly. You could start with Y9 B9 W9 K9 G9 and then sell 4 G seats (family headed on vacation and they got a good deal). That doesn't mean it will necessarily change immediately to Y9 B9 W9 K9 G5. It could be that the G inventory decreases but so do the other discounted buckets as the revenue trends for the flight are now such that the carrier doesn't want to sell as many K seats. Or you could buy a Y seat and they see more profit coming to the flight (or fewer total seats available) and they cut some of the lower buckets but leave Y at 9.

Revenue/Inventory management is half art and half science.