You can map one domain to each instance of the DNS role that you have installed within your estate.
These mappings will be included as NS records in the DNS zone whenever you or your customer creates a new website or domain alias.
For each mapped domain you will need to create a glue record with the domain registrar for the domain on which the nameservers are based.
Your resellers can do the same in order to create branded name servers.
Any name servers which do not have a nameserver domain mapped to them will still serve your zones but will not be present in the NS records of your customer domains.
To add a domain server mapping:
A Nameserver domain mapping can not be deleted because it already exists in a customer DNS Zone. However, it is possible to edit an existing name server domain mapping. Any edits made will automatically be reflected in a customers DNS zone.