For example, you will see an EV for sale with a 70kWh battery, but with a useable battery capacity of 64kWh.
The gap between the two is a buffer to prevent you from overcharging it or running it completely flat. This buffer protects the health of the battery and helps it last a longer. Sometimes you'll hear that it's best practice to only charge a battery to 80%, but there is no need for this concern in an EV, as the buffer is already built-in to make charging simple.
The car's advertised range is based on the useable capacity.