Any Clue what this pipe is for?

[LazyPool]

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]

 

[maxepr1]

Looks like the one with the 90 on it could have been discharge for backwashing at some point. The one marked "Out" goes to a vacuum?

 

[JasonLion]

The "What is this for" pipe looks like an air intake for spa jets. Or it might be a spare pipe that was run "just in case".

The "? Out" pipe is for a pressure side cleaner that does not require a booster pump.

 

[LazyPool]

Thanks for the info. About to swap out the cartridge filter for a sand filter. Cartridges are getting a bit spendy..