Please see here: https://www.davdroid.com/manual/accounts/#c272
DAVdroid address books accounts: They’re named after the CardDAV address book which they are connected to, plus a hash of the URL, for instance: My addresses (UA). The hash is necessary when there are two address books with the same name. The address book account names will be shown in your contacts app as possible destinations for contacts.
Without the hash (the two letters), it would not be possible to have two address books with the same name (e.g. “Address book”) in two different CardDAV accounts.