Envelope Follower or Ducker?

[Trebor]

I’m not sure I understand the difference between the ducking feature and autoducking, as both seemed to do the same thing.

Audacity’s built-in Autoduck only* ducks, the selected track is ducked by the track immediately below it.
So if you change the order of the tracks it has a different effect …

[ * the Dynamic Mirror plugin can duck or follow ].