Delegation
Registry servers
delegation.registry-unreachable
The parent zone is the authority on which nameservers a domain is delegated to. Comparing it against what those nameservers claim is how delegation faults are found.
A longer write-up of this check has not been published yet. The report itself shows the raw evidence behind every result, which is usually the fastest way to see what was measured.
Run this check on a domain
Registry servers is one of 56 checks in every report, alongside delegation, mail authentication, TLS and registration.