12 year old home and basement bathroom plumbing including vents was stubbed in during construction. About a year after construction the bath was finished.

Today the kids plugged up the toilet. After plunging it to clear it I went back in the storage to look for something and noticed that a small amount of toilet water including waste had come up this pipe in the low concrete area formed around my sump crock. It's simply a piece of pvc cut off at the floor that has a silver drain cap stuck in it as you see. The pipe coming up from the concrete to its right is the vent pipe that goes thru the roof.

This pipe that had toilet water come out of it....clearly it was placed in the sump area in case water came out of it and when I pull the cap off it it does have water sitting in it.

What is this pipe/drain/vent for and why is it tied into the toilet drain somehow?

Thanks in advance!

I think it's just a floor drain and the water inside is because it has a trap. The water in the trap keeps sewer odors from coming out into the basement.

So it has a small amount of sewage come out when I plunge the basement toilet maybe because the toilet is only about three feet from it?

Generally, no. If sewage comes out of your floor drain when you plunge the toilet, it is not just because they're near each other. Plunging a toilet temporarily increases the pressure in your sewage pipes. Pressure tries to equalize through the easiest path. If the easiest path is to push liquid up the floor drain, that means that the path straight down to the main sewer pipe, which is supposed to be easiest, is not very easy. I.e., it may have some blockage. You might want to try snaking the line. That floor drain may be a good place to start.