Sorry this answer is so long, but I think it answers your question... oh, and XBox360 game without "M" rating, Burnout: Paradise.
Sony has decided to use mainly the PEGI "Pan European Game Information" rating system, which allows them to rate MOST but not all games with only a GUIDE age restriction, whereas Microsoft has gone for the ESRB system which many retailers treat as legal, even though it's not!
PEGI - (Grey / White Rectangle) Reccomended but not legally required.
BBFC - (British Board Of Film Classification - The Red Circle or Green Triangle - Also rate Films) - Legally enforced
ESRB - (Entertainment Software Rating Board - Teen / Mature e.t.c) - Reccomended but not legally required, although most places treat it as legal.
For example, if you were 12, you could walk into Woolworths tomorrow and pick up Rainbow 6: Vegas 2 (PEGI: 16, ESRB - Teen) but NOT Resistance: FoM (BBFC: 15, no other ratings).
Developers can get away without a BBFC rating if they can reduce the violence in their games (I.E less gore and scary bits) Most recently, when making the PS3 game "LAIR" the developer had to remove any Blood or Ragdoll physics in order to get a ESRB rating "Teen" instead of "Mature", as they knew they would lose sales for no reason! CRAZY!
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- d23