Just a small hint as it seems that oC 9+ contains yet another hidden feature which is not documented (as quite often):
Always wondering what the following command is used for:
occ dav:sync-system-addressbook
The docs itself about it is not really descriptive:
Synchronizes users to the system addressbook
After digging around it seems this was introduced by @DeepDiver1975 in https://github.com/owncloud/core/pull/20719
However the documentation for this and the usage is completely missing. It seems one can attach an external CardDAV client to:
http://example.com/remote.php/dav/addressbooks/system/system/system
and is getting the users existing in the ownCloud installations as Contacts / VCARDs.
You can even sync these addressbooks over federated Clouds with oC 9: https://github.com/owncloud/core/pull/20948
occ federation:sync-addressbooks
Ref for a request to document this: