Haroon, BOAC is right there's no such limitation as per C. Jeppesen flight plan format contains this calculation to determine the allowable load. Many guys mistake it for a limitation and actually believe that no more fuel can be uplifted. You can add more fuel disregarding the MTOW based on MZFW provided you don't bust the usual RTOW, structural and MLW. It's a nice to know figure nothing more.
