The LIBID of the type library that the pTLB parameter points to.
Marshal.GetTypeLibGuid(UCOMITypeLib) extracts the LIBID directly from an existing type library. This action differs from that of the Marshal.GetTypeLibGuidForAssembly(System.Reflection.Assembly) method, which calculates what the LIBID should be based on the current assembly.
For more information about library identifiers, see Exported Assembly Conversion.