What are the scenarios where Apple records HRV(Heart Rate Variability) in the background?

What are the various scenarios when Apple records HRV(Heart Rate Variability) automatically in the background ?

Apple definitely computes HRV when someone uses Breathe App (in Mindfulness) on their Apple Watch and when we closely look at the details of data collected in HealthKit app, it depicts Algorithm Version as 2 But Apple also records HRV in some other scenarios which depicts Algorithm Version as 1. What are those scenarios?