"The SA standard was designed for competitive auto racing while the M standard was for motorcycling and other motorsports. The K standard was released to accommodate helmets used in karting. There are three major differences between them:

1 The SA standard requires flammability test while the M and K standards do not.
2 The SA and K standards allow for a narrower visual field than the M standard (Some SA and K certified helmets may not be street legal).
3 The SA and K standards include a rollbar multi-impact test while the M standard does not."

Some sanctions do not even allow M rated helmets for auto racing.