Looks to me like a vent? But why. Also I dont see where the main drain is directed to pump. could the skimmer and main be plumbed into each other. I doubt that?
[attachment=0:9mwnadr5]Filter In Plumb.jpg[/attachment:9mwnadr5]
[attachment=0:9mwnadr5]Filter In Plumb.jpg[/attachment:9mwnadr5]