The IPersistMessage::GetClassID method returns an identifier representing the form server that can manage the form.
See IPersistMessage : IUnknown.
HRESULT GetClassID(
LPCLSID lpClassID
);
The IPersisMessge::GetClassID method sets the contents of the lpClassID parameter to the form server's class identifier and returns S_OK. When a form viewer calls GetClassID and it returns successfully, the form is placed in the Uninitialized state.
For more information on how class identifiers are used with structured storage objects, see the documentation for the IPersist::GetClassID method in the OLE Programmer's Reference.