User Sources/Autocreation
From Authentication Tools for Joomla! (JAuthTools)
The core purpose of User Sources is to provide user autocreation services to other systems within JAuthTools (e.g. SSO). The built in login system (SSI) provides user autocreation, however this is limited and might be altered in future by JAuthTools.
In Joomla! 1.0.x, user autocreation was supplied by the LDAP Library through the populateUser call.