If you’re talking about an upgrade coupon, the maximum you can upgrade to is a Premium car. An upgrade coupon would take you from an Economy to a Compact car or a Standard car to a Fullsize car. You can’t get any higher than a Premium car, so if you book a Premium car and use an upgrade PC code, it won’t get you a Luxury car.

Upgrade coupons do do not apply to anything other than the standard car classes, so no effect on SUVs or other vehicle types.