Originally Posted by
Globaliser
Assuming that this is a cash ticket, if you use your QFFF numbers in the booking you will earn QF miles and QF Status Credits. If you use your BAEC numbers in the booking you will earn BA miles (ie Avios) and BA Tier Points. You can't mix them.
... and you can change the numbers to which ever frequent flyer programme you want that is a partner before the flight. (But do check it is in fare bucket / class that earns in that programme).
[Adding this because I've encountered people who thought they were stuck with what they put in at time of booking.]