capacitance guages are sticks of open layered "spongy" material. Each layer in the stick has a different capacitance if the sponge is full of fuel or not, so the stick just measures how much of the stick is sitting in fuel, compared to how much is sticking out of the top. The advantage is that there are no moving parts to fail, and only tiny voltages are required for operation (a good thing, when the thing is sitting in a fuel tank!)
It should also be mentioned that (on modern jets) there is a mass sensor in the fuel line leading to the engine, so the fuel flow is also in true kilos, and thus the Fuel Control Unit commands a set mass of fuel to the engine, not volume.