1. "the letter" = booking class = RBD is position 1. Position 2 is optionally the second RBD code for dual RBD.

2. Dual inventory fares means you have to have inventory in both classes, for revenue control. Otherwise you would be bumped to a more expensive fare in the same primary booking class.

3. Yes, they are airline specific. Expertflyer doesn't decode them, experflyer pulls the ticketing information airlines publish that tie with the fare code. If you know how airline constructs the codes, you can learn more than experflier shows, i.e. baggage allowance.

4. Because NDC is New. Adoption is moving slowly.