M rated helmets are rated and tested for your head hitting an object or hard surface. SA is tested and rated for having something hit you in the head. Fire rating is also included in SA as well as eye port size.

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:

The SA standard requires flammability test while the M and K standards do not.
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).
The SA and K standards include a rollbar multi-impact test while the M standard does not.