Resources
What is an SOA record?
The SOA record holds the administrative data of a zone: its primary nameserver, its contact, and the timers for transfers.
Every zone has exactly one SOA at its apex. It names the primary nameserver, the responsible contact, and five values: serial, refresh, retry, expire and minimum.
The serial is the important one. Secondaries compare it to decide whether they need a new copy — if it does not rise, a change is simply ignored, however correct it looks on the primary.
The contact is written as a domain name with the @ replaced by a dot. It should be an address somebody actually reads; every CTPFDNS zone uses dnsmaster@ctpfdns.com.
Refresh, retry and expire only matter when NOTIFY does not arrive. With NOTIFY working, secondaries update within seconds.