The cast iron vertical pipe in back of picture I think is the main sewer stack. There is something near the bottom and wonder if this is an access for cleanout and if so at which side looks like may have the removable plug. There is something right at the top but unfortunately don't have a better picture of what it is. It is inside drywall in basement and to the right is the bathroom. The ABS pipe in front I believe is the vent stack. The horizontal pipe along the bottom connecting with the sewer stack just below what I think may be the cleanout is the bathroom sink drain.

If there is a cleanout there, I can't see it in the photo.

I do not see anything that looks like a cleanout plug. In fact, according to your picture, that pipe at the bottom looks like it is connecting the two risers together, which would normally be a major no-no.

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



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Actually the angle of that picture does not show the whole story, the pipe along the bottom as it comes towards the camera turns up before it reaches the vent stack then hits a T where the sink drain connects to, then it continues up for a bit then connects at a T at the vent pipe. If that makes any sense. I posted a picture of this a month or so ago when I was having trouble snaking the sink and it first went up the vent until I got a drop head auger from advice here, then it went down and worked.

So what is this section that I circled?

That's a hub in the tee fitting. It's there for the pipe above to sit inside and then the gap around the pipe gets packed with oakum and lead. Just like the hub below the ABS pipe.