What is this thing?

[Goolsbymd]

I don't have a lot of pool experience so can't tell what I'm looking at but could it be a surge tank to reduce the sudden pressure changes from starting and stopping your pump?

[waterbear]

Looks like a Pentair Stainless Steel DE separation tank to me!

[FlatDrPepper]

Thank you all for your help! I will look at it closer tomorrow but I do have a DE filter and a separation tank would make a lot of sense since when I directly backwash I have noticed some dirt buildup on the alley behind my house where the water drains. I suppose I should have been using it all along. Oops :/

I'm still a little bugged about the skimmer/drain selector issue. Through my other readings it really seems like there should be two plainly visible pipes coming up out of the ground going into the pump. I do have a polaris pump- maybe it's somehow hooked up through that? I will have to investigate more later. Anyway, thanks again!