CAT=Central Area Turnaround.....if you arrive into heathrow and are operating another service out of heathrow if there is less than 2 and a half hours before your next flight you go to the CAT lounge which is airside instead of back to the compass centre...BA do pay you nearly 30 pounds for each visit to the cat lounge...i have a 3 day 8 sector trip next week with 3 cat payments in it....fab 90 pounds before i even start with allowances..........
It is all quite complicated all will be explained on your coarse...good luck ams-lhr!!