Originally Posted by eefor jfp

It depends on the airline. Some airlines like LH have a field for entering FQTV (earning) and FQTS (status) on the reservation. For some airlines, you can show one card at check-in for the benefits and accrue miles to another airline. Most US airlines (at least UA [and previously CO]) required the card you swiped at check-in to get the benefits be the one that the miles went to. In other words, if I wanted the free bag, I had to check-in using that card and enter the number. But it was usually possible to change it later at the gate or at the club and certainly to do it on a multi-leg itinerary for the second an subsequent connections.