What is this pipe called, and how does it work? (Round II)
Yes, it's a vent for a toilet. If it were an in-floor shower, you might see a P-trap and vent, but the toilet has the trap included in the fixture.

The reason you see the vent here and not other locations is that a sink will be vented starting in the wall rather than inside the floor, and the toilet is often after the sink, making the line from the sink to the toilet a wet vent for the toilet. In the pictured situation, the toilet is at the end of the line, and without a wet vent it needs a dedicated dry vent.