The ManagedBy element contains information on who manages this domain if the administrator has specified the value of the managedBy attribute on the domainDNS object. See [MS-ADA2] section 2.8.

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

The ManagedBy element is populated from the domainDNS!managedBy attribute on the domain NC root object ([MS-ADTS] section If the domainDNS!managedBy 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/ManagedBy element.