From The Apple Wiki

The SDOM (Security DOMain) tag specifies the device that the firmware is supposed to be ran on. It allows Apple to make sure you are not running another device's firmware files on an incompatible device, probably to prevent using old exploits. It can be found next to the ApSecurityDomain in the BuildManifest.plist of an IPSW. Every production device has the value kImage3SecurityDomainDarwin (0x01). kImage3SecurityDomainManufacturer (0x00) and kImage3SecurityDomainRtxc (0x03) are unused in public builds.