Domains are generally broken down into geographical, organizational, or functional subdomains.
Updates occur only on the master server.
The root master server is the master server for the root domain. A root replica server is a replica server for the root domain. A subdomain master server is the master server for its domain. A subdomain replica server is a replica server for its domain. The only exception to this rule, a la UNIX, is that all servers of the root domain are clients of the root domain.