What is considered a "Fuel Efficient Car"?

Yesterday, my roommate and I were shopping. We had just parked my car and we were walking into the store when we noticed that in the first three parking spots (the one’s normally reserved for handicapped parking) were signs reading: Fuel Efficient Vehicle Parking Only.
What exactly is considered fuel efficient? Is this referring to Hybrid vehicles? If you parked in this space and got a ticket, couldn’t you make an argument that your vehicle is in fact fuel efficient and that the sign isn’t specific enough?