A ZoneTransfer is how DNS servers share information with each other about a Zone that they are both authoritative for.
It is typically only permitted to trusted hosts, because while a ZoneTransfer cannot be used to inject bad data into your DNS system, it can be used to find out about the machines in a network segment even if this information isn't otherwise advertised.
5 pages link to ZoneTransfer:
lib/main.php:944: Notice: PageInfo: Cannot find action page