I remember back in probably the late 80's or early 90's, there were filters being branded as water-polishing filters. If I remember correctly, they were made by Vortex and they used diatomaceous earth to polish the water. They looked similar to a canister filter and filtered out *tiny* particles out of the water, making it super-clear, or polishing it.
The last time I saw something similar being used was on a Marineland filter, like a Magnum 350 and a H.O.T. Magnum (Hang On Tank) canister filter. They had these filter inserts that were made out of cloth or something that were used to polish the water. When the filters got clogged you could soak them in a bleach / water mixture and they would release all the particles they had captured and were ready to be reused.
I say all of that to concur with Scot in that it's a filtering mechanism, though I've always seen it in reference to a stage of mechanical filtration, not chemical or biological filtration. In my personal experience, I've only seen it used to polish water that's in a tank containing fish, it obviously took place after the reverse osmosis process.