What is the difference between Master and Slave DNS zone?
When adding a domain in client manager It is asking to add as master or slave? Can you explain what it this?
Essentially the only difference between the two is that you can add records to a Master zone, while a Slave zone can only inherit records from an existing Master zone. Both types are authoritative for their associated domains, meaning that they can hold the information necessary to associate a domain with DNS records, but they get this information differently. Master zones contain the records themselves while Slave zones get those records from elsewhere.
If you need to add a record such as an A/AAAA or an MX to a domain you'll want to set up a Master zone. Within the Domains section of the Linode Manager you'll be able to add and remove records as necessary.
If you only need to create a backup zone which gets all of it's information about records from an existing Master zone you can use a Slave zone.
Here is a guide on adding DNS records to a Master zone for your reference: