What metrics/data are used to update progression levels?

In https://support.trainerroad.com/hc/en-us/articles/4404665021595-Progression-Levels#:~:text=Progression%20Levels%20are%20determined%20by,successfully%20they%20completed%20these%20workouts. it says that

As you complete rides, Adaptive Training analyzes and adjusts these levels based on the relative difficulty of your workouts in each zone, as well as how successful you were in completing them.

How is your “how successful you were in completing them” determined? Is it based on heart rate? Your answer to the survey score?

Scroll down to the section " How Does Adaptive Training Work?"

How to Use Adaptive Training

Obviously, also in there will be some form of proprietary secret sauce as well.

If it were that simple you wouldn’t need any form of AI.