Originally Posted by Calchas

You read the table from the top down, stopping when a rule applies.

"A REQUIRED WHEN AVAILABLE" means that if the flight has A class and has space in A class, then the fare must be booked into A class (unless a rule further up the table already took precedence). If the flight has no A class, or if A is 0, then you can continue down the table.

If you wanted to restrict it book into A if A is ever sold on the flight, and not allow the passenger to downgrade into a different bucket if A is full, the wording is "A REQUIRED WHEN OFFERED".

[This is actually a human-readable interpretation of a set of logic rules. The computer doesn't read the table exactly when it wants to book a passenger into a bucket, but it gets the information from the same place as the table is constructed.]

By the way, I actually dug out the ATPCO documentation that describes the format of this table. The document is literally 272 pages long: just for the RBD table, just to describe the data format for allowing a fare to define which classes it books into. So don't feel too bad that it isn't obvious how it works.