The external user administration is opposed to the internal user administration with MySQL in contrast to the internal user administration. If you log on to your ArchivistaBox, for example, you access the MySQL database directly, i.e. you work internally.
If a central user administration exists (e.g. LDAP / ActiveDirectory), the ArchivistaBox can also be connected to this 'center'. I.e. each time you log on, WebDMS checks the central computer to see whether the account used exists and is valid. Only if this is the case does a logon take place.
When logging in, an account is created on-the-fly in ArchivistaDMS using the information from the external server, whereby Archivista-specific fields of the user account are taken from a user specified as a template and already existing in Archivista.
You can define the way you want to handle your user management under this menu item, as well as the settings required for it.