It's a casting number to order the brakes with to get the larger Tundra brakes...the 231 mm...199 is just a little smaller but since I'm running 18 inch wheels I can fit them no problem...I got all my brakes from one seller on Ebay called Brakemotive76...I got the powder coated calipers, crossdrilled and slotted black rotors and ceramic pads for 300$ delivered...I use these guys for most brake jobs I do weekly...I don't do links because if they change the sale page the link is no good...can search the seller up then search the store for tundra brakes...they got 2 sets to choose from...there's a couple great write ups in this site for the brake upgrade.