1st summer as a pool owner- can't figure out what the 4th ball valve does

My husband and I bought our house Sept '14. Previous owners gave us a 2 minute rundown on equipment and maintenance as they picked up their last bit of belongings. Left us everything for maintaining pool...nice people.

Anyway, we've never had a pool and swam maybe 2 times before weather changed and it was time to close up the pool. Had a guy come winterize and had a safety cover installed in November '14.. Light went out in May and I replaced it myself!!! That was a proud moment for me. I also replaced a rusted out filter pressure gauge. I have spent hour upon hour researching and using trial and error to figure out how everything works with the pool equipment, chemicals, etc. I was thrown off for months on which ball valve does what, but finally figured there was a clog from my skimmer to the pump. After the clog issue was cleared up, I now know one ball valve controls the skimmer, another controls the floor drain, a third controls the jets near my steps, but there is a 4th ball valve that is connected to a separate pipe that connects to the pipe going to my pump (the one for the floor drain and skimmer) but I can't for the life of me figure out what it does. I'm on my phone and can't update my profile, so here's the stuff I know about the pool:
18x36 rectangle in ground with vinyl liner.
3ft deep in shallow end, 8 feet in deep = approx 25-26K gallons
Sand filter with automatic tablet chlorinator connected. There's a pump, don't know anything about it, except it works. My pump is controlled by what looks like a regular breaker box. I don't have a timer on it or anything. It's either on or off...wish I had a timer. The pool was completed in either 1994 or 1997, but electrical is new. Had an electrician out when I first bought the place and he knew our house, said he'd redone the pool electrical not long ago. Vinyl liner has tons of places where it's creasing. Creased bug me, but pool guy who winterized it said it had another good year or 2 left so I said if it isn't broke I'm not fixing it!

I didn't think to take a pic of the valves or I'd show you the setup. Will post follow-up pic tomorrow.

Any idea what the 4th one could control? Also, there's a connection near my skimmer in the pool wall that has a small cover screwed on. I can unscrew it and it looks like all the other jet lines except no water flows from it...no suction either. Mystery valve does not control it... What's it for? Previous owner left me a kreepy krauly; instructions for it say that wall line is generally an additional suction. Any ideas how to turn it on...if that's what it is?

Thanks in advance for any help. Oh yeah, I live in Louisiana where we're barely above the water table. There's a sump pump near the back of our property that removed water so it doesn't build-up behind the liner. Imagine my surprise...first good rain we had after buying the house, I thought we had a tiny geyser on the back of our property. :)
 
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