That is correct. So if an intra-US flight has I Booking Class, then it must be used. If not, then an upgrade to First (A) is permitted. If there is no A Class either, then there will be a downgrade to Economy (Y).
Looking back at the top row, it says "J/R REQUIRED WITHIN EUROPE". Why not "J" only, or is this shorthand for "J or R" equally or "R, but otherwise J if R is not open"?
