This field is the default number of seconds to be used for the time-to-live field on resource records that don't have a ttl specified elsewhere.
There should only be one SOA record per zone. Example 28-3 is a sample SOA resource record.
;name class SOA origin person-in-charge doc.com. IN SOA dnsmaster.doc.com. root.nismaster.doc.com. ( 101 ;Serial 7200 ;Refresh 3600 ;Retry 432000 ;Expire 86400) ;Minimum )