Its just a brand number. If it used to mean something, it doesn't anymore.
Ford has several classes of full size trucks, with weight and towing ratings based on engine, transmission and platform:
F150: GVW 5060-5910 lbs, GCWR 6650-8200 lbs
F250: GVW 5823-6409 lbs, GCWR 8800 lbs
F350: GVW 5500-6490 lbs, GCWR 9900-12000 lbs
Info on F450-F750 trucks can be found
HERE
GVW = Gross Vehicle Weight - weight of the vehicle only.
GVWR = Gross Vehicle Weight Rating - highest allowed weight of vehicle and cargo combined.
GCWR = Gross Combined Weight Rating - highest allowed weight of vehicle, cargo, and trailer combined.