The letters refer to the fare basis, where P, F, A, Z are first class fare types, J, D, C, I and U are business class fare types, and all the others are economy class fare types (except for BA where they also have a premium economy called World Traveler Plus which is W and R I think).

The number is a range from 0 to 9, reflecting the number of seats available for sale on that particular sector. The number is the minimum number of seats for sale in that fare class. So a 9 means there are at least 9 seats available for sale.

Some airlines use 9 as the highest number they show, others use 7 (like AA) or sometime 4 (often for codeshare flights). But these just mean there are at least 7 (or 4) seats available.