The energy label is needed for new passenger cars with a total weight of at most 3500kg with at most 9 seats, which can be run completely by fossil fuels. For electric vehicles, pick-ups and trucks, campers as well as motorbikes no energy label is required.
A vehicle using little fuel compared to its weight is called energy-efficient. The categorisation in energy efficiency classes ranges from A to G, A corresponds to a "very energy-efficient" vehicle. The energy efficiency label is based on a formula which factors in fuel consumption in kilograms and the vehicle's weight (for further information see Consumption catalogue by TCS).
Diesel vehicles often appear in the higher categories since their energy balance is better. But note that this doesn't necessarily mean that they are also eco-friendly.
You'll find two examples of energy labels above, the first one comes from an energy-efficient car, the latter from an inefficient one.