For which two-wheelers is a helmet mandatory?

What does this rule or driving tip mean?A helmet only works properly when it fits well and is fastened. A helmet that is too loose can shift or come off in a collision. How to apply this safelyCheck the approval mark and correct size. Fasten the chin strap before setting off. Do not use a helmet that has been through a hard fall or collision. Driving lesson and test tipGive two-wheelers extra space. Their rider is protected by clothing and a helmet, but remains far more vulnerable than you are in the car. Remember: Wear an approved helmet, fasten the chin strap, and replace a helmet after a hard impact. See the source: RVV 1990, article 60.