This is the first time I have seen this in a basement floor. The 9" dia. pipe is about 3’ from the sump pump and the lid reads (Backwater valve access sleeve.) I removed the lid an down about 12" is what looks like a plastic plug. It this to prevent sewer water from backing up into the basement drains?
Yes, that is the function of a backwater valve. The access cap, in your picture, I think, is a cleanout in case something jams or gets stuck in the flapper, in which you could remove obstruction- which is all it is, a flapper that lets wastewater out,then closes to prevent backups.
Thanks for the info!!, Ken