[3][4] A domain is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.
Some network administrators took the additional step of having a dedicated BDC online for the express purpose of being available for promotion if the PDC failed.
In Windows NT 4, one DC serves as the primary domain controller (PDC).
[6] The BDCs exist in order to provide a backup to the PDC, and can also be used to authenticate users logging on to the network.
In modern releases of Windows, domains have been supplemented by the use of Active Directory services.
The same rules apply; only one PDC may exist on a domain, but multiple replication servers may still be used.
[7] Primary Domain Controllers (PDC) have been faithfully recreated on the Samba emulation of Microsoft's SMB client/server system.
In Active Directory domains running in native mode, the concept of the PDC and BDC do not exist.
A side effect of this change is the loss of ability to create a "read-only" domain controller.