Basement Tub Rough In

I just moved into my new home and I'm not wasting anytime starting on the basement and full bath.
I'm stumped. The builder did a good job on the rough in for the tub, toliet, and sink, but there is a pipe sticking out the concrete right next to the tub rough-in that I can understand.



I found the floor plans, and they show two different plans for the basement bathroom. Do you think someone messed up and put that pipe there by mistake and then roughed in the tub drain?
Any help here would be extremely appreciated.

pipe

It would be more likely that it is the vent for the bathtub drain, but without more information, that is just a guess.

what hj said. Vent.

David
p.s. do you know you'll need more than one vent?

My ignorance!

I thought about that, but it is really close to the wall, so I threw that out.
What I need to do is just buy the shower I want, then it will all come together.
Give me electrical, heating, framing, and sheetrock and I'm fine, but I'm out of my element with plumbing.