Thanks everyone for all this extremely interesting information!

Here are some questions, if anyone is willing to spend the time answering them:

1) If "the letter" is RBD (position 2), then what is the booking class (position 1)?

2) I don't understand "dual inventory fares" -- what's the point? Is this about having a domestic/intracontinental feeding into an intercontinental or something?

3) So fare basis codes are airline-specific, and you need something like expertflyer to decode them (i.e. to know the interpretation of the codes for each position)? How does expertflyer get the information?

4) If NDC is better than GDS, then why doesn't everyone use the former? Is the software more expensive/proprietary? Is the per-access cost higher?