That meens, there is a heterogen distributed Userdatabase.
All Authentication-Systems like radius, Tacacs, and login/ftpd/... have to ask the Database.
So the passwd-Map of the NIS/YP can be replaced by the central Usermanagement database.
In the Ascend/Livingston-Implementation, there is a Default-Entry,
which is taken, if no other userprofile is found.
By W-4 there is the policy that people
have a free E-Mail-address (that means, the have a E-Mailaccount/login)
but are not dail-in-customers.
The users-File in the other implementations are like an logic-System: If you have this Check-attributs, you get these Reply-Attributes.
Some Reply-Attributes may be nonterminal. So we can have something like groups or common Attribut for an NAS or Service.
The Reply-Attributs can be regarded as a Capability in object-oriented Operationsystems like HYDRA.