For reliable operation of domain, it is necessary to have at least two DNS-nameservers. These servers respond to all requests to your domain (extension) on the Internet.
DNS-nameserver storing all initial information is called Primary DNS-server (master). And it is called Secondary(slave) if it receives full information about the extension from the Primary or another Secondary DNS-server.
In order to activate this service just check out “DNS Management” check-box during the domain registration procedure.
After the order has proceeded, DNS-nameservers of HostZealot would be set as primary and secondary for the client's domain. And the client could manually add A, CNAME, MX, and other records to the domain (the number of such records is unlimited).