Help identify a valve

[nels864]

Can someone help me identify what the circled valve in the attached diagram does? I bought this home a year ago and can't determine the purpose of theis valve. Thanks in advance.

 

[Bama Rambler]

It has to be a suction port somewhere in the pool. Look for a wall suction somewhere.

 

[257WbyMag]

That's very strange. If there wasn't already a "pool vacuum" port identified already, a wall suction vacuum connection would be what I would guess. But one of the valves clearly says "pool vacuum" already.

 

[Richard320]

Any chance it goes to the main drain?