I’ve had a bit of inconsistency getting my DL Platinum number into my AF bookings (which have been primarily domestic French flights), but if I couldn’t do it online, I’d call in.

I did have once where I spent almost two hours on hold with about 3-4 different agents before someone managed to get the number placed in… eventually it was a Flying Blue specialist who managed to enter my Delta number. It was rather frustrating and never really explained why… But after that, I was able to select any seat including the premium ones free of charge, and never had a problem with an extra bag either.

It bears noting also that in the booking process, I usually was able to enter the number, but for seat selection I was being asked for additional money (which I knew wasn’t right). Sometimes all it took was completing the booking process, and then selecting a seat if the number was already recorded. Other times, I’d have to put in the number again online after booking, and then I could select seats without charge. Only once did I have the hassle of having to ring up Air France and wait through several different holds with several different departments before it was resolved.

I guess, in summary, it’s very “glitchy”