What is Source=api4 in Looker System Activity Explores?

Trying to find some information on what the values in the History Source field in explore/system__activity/history Explore represent. Specifically want to know what it means Source=api4. I thought that would mean queries run via Looker API, but I’m sure these queries associated with users who have never used the API directly. So I’m wondering what kind of user action does Source=api4 represent?

I guess same question goes for all possible values in History Source dimension