SoftLayer_Dns_Domain_ResourceRecord_SoaType defines a domains' Start of Authority (or SOA) resource record. A domain's SOA record contains a domain's general and propagation information. Every domain must have one SOA record, and it is not possible to remove a domain's SOA record.
SOA records typically contain a domain's serial number, but the SoftLayer API associates a domain's serial number directly with it's SoftLayer_Dns_Domain record.