It appears to be teed into the suction side. Do you have two skimmers?
The other possibility is that someone upgraded and added a second main drain and that's where they tapped in. It does look like sort of an afterthought.
Currently that side of the suction is shut off. Move that valve 180 degrees and see if the flow from the skimmer stops but the pump keeps working. That will indicate there is a main drain down there. It might help you clear things faster if you have some suction down below.
I do have two skimmers, I have the main drain closed and skimmers open because I believe the main drain is clogged with so much debris at the bottom of my pool. My filter stops shooting out the returns after about a hour., and i have to back wash to get them going again
It looks to be an air vent and if you don't have a spa, it could be a SVRS line for the main drain. If the main drain gets blocked, this line will allow the pump to suck air and lose prime so the main drain no longer has any pull. Is the main drain line right behind it?
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