Soil stack vents.

From the Kaplan Building Systems ARE study guide:

"In the soil stack, the section above the highest fixture is called the stack vent (as opposed to the vent stack) and vents the soil stack.  The vent stack is a 'stack' of vents; the stack vent is something that vents the top of the soil stack."

"Stack" and "vent" are starting not to look like words.