Looks like an overflow drain, but no one could make sense out of it without seeing the surroundings.

Yeah, a look at the surroundings would help.

Looks like a drain, but usually they're located down low so they can do their job.
Have you taken the top off and looked? It could have a valve down there, but I doubt it.
Does that area look like it needs a drain? Is there an outflow drain anywhere?
Mike

Looks like a vent to me. To provide air to drainage below to keep it flowing?

Looks like a vent to me. To provide air to drainage below to keep it flowing?