Users Runtime Configuration - Unclear distinction between User Manager and Rbac Manager

I've consulted the documentation located at:
https://archiva.apache.org/docs/1.4-M4/adminguide/redback-runtime-configuration.html#Apache_Redback_User_Manager_Implementation

However, it's a bit unclear as to what the distinction between the User Manager and the Rbac Manager. I know that Rbac handles security for Archiva, but that implies the users as well. So, why are both listed?

If anything, some hints should be added to the Archiva page and also better explained within the documentation.