Please see https://forums.bitfire.at/topic/1674/what-is-the-2-letters-after-the-account-name-for-address-books and the links there (especially the manual)
I am using DAVdroid fo sync my Android contacts against a horde groupware server.
After starting the DAVdroid app on the smartphone I see 2 addressbooks with the same name. But I have only 1 addressbook on this account on this server …
Upgrading to Horde Framework 5.2.14 solved the problem.
Thanks for the tip.