It used to be that if you made an AAdvantage booking on QF, you'd have to phone up to obtain the QF PNR (AA doesn't bother to give it to you) in order to do seat allocation. If you wanted the booking to be visible in your booking list on qantas.com, you'd have to email AA to get them to add your QF number to the booking. (You couldn't add it online on either site.)

The first of those requirements disappeared a while back. qantas.com these days understands AA PNR codes as well as its own.

Now, the second one is unnecessary too. If you are logged in, you can display your booking using the AA PNR, make seat selections, and your QF number is automatically added to the reservation.