The SYSTEM location contains a database called XDBACF (XDB Server Access Control Facility), which contains the following tables:
Contains a list of all users, including AuthIDs and Passwords for each user.
Contains a list of all GroupIDs that have been defined.
Contains a list of users and the GroupIDs they are authorized to use. Each user (Primary AuthID) may have several entries, one entry for each group ID that the person is authorized to use.
See the SQL Reference for detailed information about the contents of these tables.