1- It's an ECAM caution MSG
FUEL FU/FOB DISCREPANCY
Difference between initial FOB and current FOB + fuel used data is more than 3500 kg
-FUEL LEAK ............................... CONSIDER
2- My understanding is that difference of 3500 kg has been elected to allow an acceptable level of errors in the indicating system in regard of these huge fuel tanks.
3- That makes sense.