Some sanctioning bodies require this decal to mark the location of your transponder. Our 3 inch diameter red-on-white "T" decal has a special area for you to write in your transponder number, if desired. This "Write-on" area has a slightly textured surface similar to the signature panel on a credit card. You can enter your transponder number using a ball point pen or "Sharpie" type marker. Make sure that the ink is waterproof if the decal will be exposed to the elements. If in doubt, you may want to laminate the "Write-on" area with clear tape to protect the transponder number from rain.