5. Any Tamiya Hop-Op Option and spare part is allowed as long as it is used in the way it was designed, and on the vehicle it was designed for. Except for the GT1 class, it is strictly forbidden to cut, mill or lighten the chassis. It is strictly forbidden to re-engineer or modify the cars in any way other than with Tamiya Hop-Ups. (This applies to all classes)
i think it refers to the "designed for". if i remember right, the packages have to list the chassis it is being used on.
im not sure myself, really... would be best to email TamiyaUSA and verify.