A canister filter is a closed system, so the gravity pulling water down on one side is counteracted by gravity on the other side. (sides being input and output from the canister) So, gravity is effectively taken out of the equation. "head" is really the weight of the water due to gravity. So, in a canister filter, there is no "head", only resist of pushing the water through the filter material and limitations of water flow due to the diameter of the intake and return tubes.
In a wet/dry filter system or sump filter - gravity pulls the water into the sump tank. The sump tank is open, not airtight. Therefore the pump which returns water has to fight gravity. The force of the water coming into the sump tank does not help the water get back up to the main tank. (like it does in a canister filter) Therefore, the return pump has a "head" equal to the difference in the height water levels of the sump tank and the main tank.