These parameters (located in Service Plans > select a plan > DNS
tab) define how the DNS service running on the Plesk-managed server will
serve DNS zones for websites hosted on the plan.
Master
A master or primary name server stores locally the zone file it serves,
while a secondary server only retrieves a copy of this file from the
primary.
Slave
A slave or secondary server retrieves a copy of the zone file from the
primary name server.
