People coming from ILSAC GF-4, GF-5 somewhere in their cars' manuals or similar need to know if they can use GF-6. They can (GF-6A being "backwards compatible") and they cannot (GF-6B because of lowered HTHSV not being "backwards compatible"). That's all.
Exactly ... it's a viscosity compatibility (HTHS) issue, not a formulation issue within the 0W-16 viscosity rating.