OK, thanks for people's help on my last question, now I've confused myself again.
I came across this picture, similar to the last question, but I don't think it's a cleanout. It looks like a vent to me. I'm referring to the part in the red border but I've included the rest of the picture for reference.
Am I correct in saying this is a wet/dry vent to equalize the P-trap from the closet? If so, I haven't seen many installations like this, why is it done this way? (more specifically, I thought vents were usually downstream of the flow…?)
EDIT Well, I've answered the easy part of it – if it is indeed a vent, then anything after the P-trap in the closet is technically "downstream of the flow".
Best Answer
Yes, it's a vent for a toilet. If it were an in-floor shower, you might see a P-trap and vent, but the toilet has the trap included in the fixture.
The reason you see the vent here and not other locations is that a sink will be vented starting in the wall rather than inside the floor, and the toilet is often after the sink, making the line from the sink to the toilet a wet vent for the toilet. In the pictured situation, the toilet is at the end of the line, and without a wet vent it needs a dedicated dry vent.