What's the purpose of a 2nd soil stack?
I was looking over the DWV pipes in my basement and realized I have 2 soil stacks (if I am using the term appropriately). As you can see, there is the main 3" one on the left, but there is this separate 2" one on the right. I tried to trace the lines, but can't really tell what the 2" line goes to. I have a 2-story house if that helps - the 2" line heads upstairs but I have no idea why I would need that separate line right there and can't think of any drains in the vicinity where it goes into the next story.
The other weird thing about the 2" line is the short stub-out you can see in the left pic - that is in the joist space.
Any ideas what this 2" line might be for, and why it has that stub-out?