The different ways rental companies handle fuel, and which is fairest.
The most common and generally most favorable policy is "full-to-full": you receive the car with a full tank and return it full, avoiding any markup. Less favorable options include prepaid fuel plans, where you often end up paying for fuel you didn't use.
Confirm your specific policy at pickup rather than assuming, since it directly affects your final cost and the paperwork you'll need to prove compliance at drop-off.