DNS - What is a delegation zone
For a DNS server to answer queries about any name, it must have a direct or indirect path to every zone in the namespace. These paths are created by means of delegation. A delegation is a record in a parent zone that lists a name server that is authoritative for the zone in the next level of the hierarchy. Delegations make it possible for servers in one zone to refer clients to servers in other zones.
Links:
https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/reviewing-dns-concepts