comglue

A glue record is the address the parent zone publishes for a nameserver that lives inside the zone it serves. When it is missing or stale, resolution still works — until it doesn't.

Walks from the root servers and interrogates every nameserver in the delegation directly.

What this checks that a normal lookup cannot

Missing glue
A nameserver inside the zone it serves is unreachable without an address published by the parent. Resolvers hide this whenever another nameserver answers first.
Lame delegation
The parent advertises a server as authoritative; the server does not claim authority. Every query that lands there stalls until the resolver retries elsewhere.
Parent/child NS drift
The delegation and the zone's own NS records are published separately and drift apart. Resolvers cache the parent's list — so the server you added inside the zone may never be asked.
SOA serial divergence
Different nameservers serving different versions of the zone means a transfer is failing. Which answer a client gets becomes a coin flip.