Off is off. Noise cancellation, transparency mode, and supposedly ambient EQ is all disabled.
Off sounds like what’d you expect when you wear in-ear headphones. Your ears are just plugged up.
Transparency mode allows sound like ambient noise to go through the headphones by using the microphones. You’ll hear more clearly instead of having your ears plugged up. And the feature becomes useless if you increase your volume over 70%.
It’s only good below that and also good for hearing your surroundings.