All it does is pass water through the media. "Reactor" is a pretty fancy name... it doesn't really live up to it.
Water is pushed down to the bottom, then allowed to slowly move upwards through the media then out. A phosban reactor is really only about 5 plastic parts - a body, lid, a standpipe, a little plastic disc full of holes, and a sponge of the same size.
You don't need a sump to run one. The namesake product (Two Little Fishies Phosban Reactor) product can be set up as HOB, you just attach a powerhead to the intake and dangle it in the tank.