@Undo basically the example is this:
example.com is on Google DNS, but sub.example.com has its own authoritative Nameservers at dns.googleplex.example. DNS entries for example.com need an NS record added as follows:
`sub.example.com IN NS dns.google.plex.example.`
This says to all DNS servers "I am authoritative for example.com, but sub.example.com is actually over on this other server."