While I think I've pretty well figured out our pool plumbing, I just discovered an in-line ball valve in our deck that I believe must be part of the suction side of the plumbing for our pool. I have no idea what this valve is for. On the suction side, there are three suction lines at the equipment pad... skimmer, spa drain, and cleaner line. There are two Jandy 3-way valves, one of these is to control pool vs. spa suction (that connects to the inlet of he pump), and on the main pool side of that is a second 3-way valve that balances the suction from the skimmer vs. the cleaner line. However, I can't figure out the purpose of the deck 2-way (inline) valve.