Using private nameservers

It is possible to delegate subdomains of your domain to alternate nameservers and that is what the Private nameservers are for.  

This is NOT the same as "Nameservers" where you can select to use custom nameservers.

Very few clients need to use private nameservers and unless you know exactly what you are doing, you probably want standard "Nameservers" option instead.

To set up domain delegation, use the record type "NS" in the zone editor.