On the A321neo, the QRH has a procedure in the case of a ACT (Additional Center Tank) System Fault (Found in the QRH under Abnormal and Emergency Procedures, Fuel).
When such a fault occurs, you must land within the stated time in the following table, depending on the number of refueled ACTs at dispatch. My understanding is that ACTs are refueled in the following order: AFT1, AFT2 and FWD. They are then used, in flight, in the reverse order: FWD, AFT2 and AFT1.
According to the table, if you have refueled just 1 ACT (which implies you only refueled AFT1), you need to land within 120 minutes of the fault occurring. If you refueled all 3 ACTs, and AFT1 ACT XFR fails (which implies FWD and AFT2 ACTs are empty, due to the order of fuel consumption), you need to land within 200 minutes of the fault occurring.
Where does this difference of 80 minutes come from? In both scenarios, the only ACT with fuel in it would be AFT1. So why would you be given more time to land the aircraft in the latter scenario?