It's the last six digits of your VIN number, from your title. They are trying to determine if you have 22 mm thick front rotors or the later 25 mm thick rotors. Check the rotor listings at Autohaus for more info. If you have 22 mm rotors it should say somewhere on the rotor "min. 19 mm". If you have 25 mm rotors it should say "min. 22 mm". You need to measure the thickness of the rotor anyway - if it's significantly more than 22 mm you have the thicker, later rotor.