There is a difference with California legal and Federal legal emission requirements. However if your Ford Focus is equipped with all the Federal required emission components (which it was originaly equipped with) it can pass the California smog check and register with the CA DMV.
The rule is any USED vehicle which is equipped with the necessary Federal Emission Components is eligible for California registration as long as the vehicle's exhaust emissions fall within California's limits and the vehicle's emission system has not been tampered with. This does not apply to new vehicles. If you had just bought this Ford Focus from the dealership (a 2015 model) and the emissions sticker said it was not California legal, you would not be able to bring it to CA.
