Two pipes with almost the same name — one is the drain pipe itself, simply extended through the roof; the other is an entirely separate pipe running alongside it.
Spend enough time around DWV drawings and you'll hear "vent stack" and "stack vent" used almost interchangeably — same two words, just swapped, so surely it's the same pipe with two informal names. It isn't. A stack vent is simply the soil or waste stack itself, continued upward past the highest fixture drain connection and out through the roof — the identical physical pipe, just extended, with no second parallel run anywhere. A vent stackis a genuinely separate, dedicated vertical pipe that runs alongside the soil or waste stack for its own purpose, tying into the drainage system near the base and again near the top. One is an extension of an existing pipe. The other is a second pipe that doesn't exist in the first configuration at all.