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".