I asked a GA in ARN handling the ARN-ORD flight if there is any accounting about what op-ups they give. She said the aim is to get the flight out on time, and if there is a oversold situation they op-up by some rules. Each op-up has to be approved by the GA supervisor. They have a little talk when the 2-3 GAs meet to open the gate. They try to pick EBS,EBG,*G and *S, but when things get hectic, especially when some one is sitting in a bulkhead they need for pax with children and status ax have already boarded, the rather op-up whoever is in the seat they need, rather than go back into the plane and find the EBG.