The ObjectGuid element contains the GUID representing this domain.

 <xs:element name="ObjectGuid" type="ser:guid" />

The ObjectGuid element is populated from the domainDNS!objectGUID attribute on the domain NC root object ([MS-ADTS] section If the domainDNS!objectGUID attribute is not present or cannot be read due to the client lacking access rights to read the attribute, the server populates the ActiveDirectoryPartition /ObjectGuid element with a value of 00000000-0000-0000-0000-000000000000.