The PDCEmulator element contains the FQDN (1) of the server holding the PDC Emulator FSMO role for this domain. See [MS-ADTS] section 6.1.5.

 <xs:element name="PDCEmulator" nillable="true" type="xs:string" />

The PDCEmulator element is populated from the server!dnsHostName attribute on the parent of the PDC Master's nTDSDSA object. The PDC Master's nTDSDSA object is the object referenced by the domainDNS!fSMORoleOwner on the domain NC root. See [MS-ADTS] section for information on the nTDSDSA object. See [MS-ADTS] sections and 6.1 for information on FSMO roles and the fSMORoleOwner attribute.  If the domainDNS!fSMORoleOwner or server!dnsHostName attribute is not present or cannot be read due to the client lacking access rights to read the attribute, the server returns a null ActiveDirectoryDomain/PDCEmulator element.