16mm triple square socket with a 1/2" drive, also often called a 12 point or xzn socket. The service manual refers to it as tool number 2567 and/or 3357.

Applications where it is needed:
 - Mk5 and Mk6, R32 and Touareg only:
   - removing the rear calipers when changing brake pads or rotors.

 - VW, Audi, and Porsche Transmissions that have a tamper proof drain plug, it is needed to remove the drain plug when changing the transmission fluid.