The protected members of System.Xml.Serialization.XmlSerializationWriter are intended for use only by derived classes that are used internally within the .NET Framework XML serialization infrastructure.
SOAP encoding allows a unique XML element to appear once within a SOAP message as a multiRef element and to be referenced any number of times by substitute elements. The XmlSerializationWriter.TopLevelElement method helps keep track of the objects that can be serialized as multiRef elements.
SOAP encoding is described in Section 5 of the SOAP 1.1 specification.