What is Backup (Slave) DNS?
Backup DNS explained.
Backup DNS is also frequently called Slave DNS, and it holds a copy of all the information for a domain, including DNS records. That copy is received right away from the Master or Primary DNS. You are not able to perform any modification, and it acts only for storing the data. In case you desire to make some additional changes to the data, you should make them in the Primary DNS. Then they are going to propagate to the Slave DNS as well.