"Snell "M" (Motorcycle) rated helmets are designed for motorcycling and other motorsports. They offer slightly less protection than SA rated helmets.
Key differences between M and SA Rated Helmets (from Snell website):
SA standard requires flammability test while the M standard does not.
SA standard has rollbar impact test while M standard does not.
SA standard allows narrower visual field than M standard (Some SA helmets aren't street legal)."
pulled that from
Snell M2005 Helmet From SoloRacer.com