Previosu Employment content type - what is the criteria used from Core HR

Content

Hi,

We are using the seeded Previous Employment content type and apart from user entry, this also seems to be automatically updated from core HR with rows based on some assignment changes. 

I have checked and

1. there is no real pattern from assignment changed based on employee job ( we don't use positions). I have examples where just work measure changes also feature as a change

2. Not every date tracked change on assignment doesn't come here as well.

Is there a known trigger for the type of assignment changes that populate this previous employment content type?

Many Thanks

Nirupa