RFC 4122 defines a UUID. A GUID is the same as a UUID in string form, but differs in binary layout. Microsoft operates with GUIDs but confusingly labels them as UUIDs in their documentation and include files.
i.e. rpcdce.h: typedef GUID UUID;
The binary form matters since it is used over the wire. Mapping a GUID to a UUID (or vice versa) without the layout conversion results in structures with mangled RFC version info and different string representations on Windows and non-Windows systems.
Proton currently only calls Microsoft APIs to obtain the string representation of the UUID (which will match the GUID), so it is unaffected.