Where does Nextcloud get my contacts from?

I just installed Ubuntu & Nextcloud on a clean system.

I was toally surprised where all my contacts came from in the contacts section?? What is z-app-generated??

Ok, found it out on my own. I forgot that I transferred an ics file for the calender. There was one appointment with a lot of attendees, including their email adress. Those emails have been imported automatically from the calendar.