Originally Posted by AK370Z

Small round button above the rear license plate is to lock unlock your car. I use it all the time. It's very convenient if you're going to the trunk first and then inside the car or you're simply getting something out of your trunk. Since this button unlocks the car, your fuel door is unlocked during the process. I don't think it's there for fuel door.

Every time I go to a gas station, I turn the ignition off right away (in NJ here, full service). Or else I get the "why won't this guy open the gas door" look from the attendant.