SendXpsUnknown method sends an XPS document part that cannot be identified to the filter pipeline.
HRESULT SendXpsUnknown( IUnknown *pUnknown );
A pointer to an unrecognized document part interface.
SendXpsUnknown returns an HRESULT value.
If the IXpsDocumentProvider::GetXpsPart method returns an object that the filter cannot identify, the filter should forward the unrecognized object to the next filter in the filter pipeline by calling
SendXpsUnknown. Passing unrecognized objects to the next filter helps the filter maintain compatibility with future document formats.