Ok, this is a new one to me. This is in the floor of a basement near the sewer clean-out. Buyer was told this was a “Flood valve” on the sewer main. he understood that to mean “open this in case of a flood”, which made no sense to me.
That is a manual type floor control valve. the valve needs to be closed by the homeowner during a water event that may cause flooding in the basement. The property does not have an overhead sewer system this type of valve is common in the Chicago area.