I think car manufacturers in most cases use California emission standards when they produce a vehicle...the tag identifies this.

It's also one less worry among dealerships when they ship cars from one state to another.

Vehicles without the sticker would be considered "grey market"...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car