In most cases it is full/full, so you get the car with full tank and return it full as well.
Some car rental companies (especially in the United States and Southern Europe) require the tank on return full/empty. In this case you receive the car with a full tank but you need to return it empty as you will not be refunded for the extra petrol left in the tank. Ask at the pick-up counter which policy applies for your specific car.
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