I'm familiar with vents, but indoor vents are basically one-way air valves. I don't know if this is required by code, but it's definitely common practice (you don't want poop smell coming out of your vents)

Vent stacks need to be above that roof’s line, but your building might be next door and taller with open windows downwind.

Or a crack in the stack.

That isn't a good explanation, since venting should exit far above the building.