If you have elite status on the airline you are flying with, but want to credit the flight to another FF program, it might be useful to book the flight giving your elite number on the actual carrier. You will receive higher waitlist priority etc. You can always change the program your miles will be credited to before boarding the plane, i. e. at check-in, at the lounge or at the gate (latest option). I usually make sure at the gate that the flight is actually credited to the program I want and that (if possible) the boarding pass indicates that (e. g. a printed on LH *G if flying United and crediting miles to my LH account).