Parent returns a referral
delegation.parent-refers
A correct parent answers with AA=0 and the NS set in the authority section, which is how resolvers walk down the tree.
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
Parent returns a referral is one of 49 checks in every report, alongside delegation, mail authentication, TLS and registration.