You are buying a pack of 4 compatible polycarbon filter cartridges to fit the Fluval U3 internal filter.
This is not a Fluval Product. This is a Finest-Filters alternative which serves the same purpose as the original brand.
These filter cartridges have a polyester side for mechanical filtration, and a carbon impregnated side for chemical filtration of the aquarium water.
These filter pads are made to fit the Fluval U3 range of internal filters.