When creating a zone by zone transfer, AXFR is used. For secondary zones on our system, there are three ways to update.
Notify
When a notify is received by an active notify server [at least one of 54.217.202.161, 107.21.214.87, 54.245.253.13], one of our xfr servers [23.21.200.163, 23.21.206.251, 50.112.240.144, 50.112.240.145, 176.34.183.208, 54.75.253.83] will contact the primary name server send a SOA request.
If the serial is not newer, we will not update. If the SOA serial is newer than we have at secondary, we make an IXFR request to the primary. If the serial change is recent, we will do a partial zone transfer. If the serial is so out of date that there is no change list, we will perform a full transfer.
Refresh
When the SOA refresh expires, one of our xfr servers [23.21.200.163, 23.21.206.251, 50.112.240.144, 50.112.240.145, 176.34.183.208, 54.75.253.83] will contact the primary name server send a SOA request.
If the serial is not newer, we will not update. If the SOA serial is newer than we have at secondary, we make an IXFR request to the primary. If the serial change is recent, we will do a partial zone transfer. If the serial is so out of date that there is no change list, we will perform a full transfer.
Force
When a force is initiated via one of our user interfaces, one of our axfr servers [23.21.200.163, 23.21.206.251, 50.112.240.144, 50.112.240.145, 176.34.183.208, 54.75.253.83] will contact the primary and perform an AXFR. (In the force scenario, AXFR will be performed regardless of the SOA serial value.)