The section on the right is a refugium. In order to control the flow through the refugium and to keep fuge water out of the skimmer, and vice versa, you split the drain line so that most of the water enters by the eggcrate, where the skimmer is plumbed, with the fittings on the left. But some of the water goes direcctly to the fuge, which then drains into the central section, along with the bulk of the drain water from the tank and skimmer. Then, instead of putting a bulkhead on the center section in the front and running PVC pipe to the return pump, I put this pipe so that water from the center section can exit the side of the sump, where there's a fitting to plumb the return pump. This way there's no need for plumbing in front of the sump. (you're actually looking at the back of the sump in this photo) This let me build the thing the full front to back width of the stand.