Nikon CLS group C?

[c22w1]

I can't seem to find any reason why my Nikon speedlights have 3 groups (A,B and C) but both my Nikon cameras, used in commander mode, can only control 2 groups ie A and B.
What is the purpose of group C on the speedlights please?

 

[magicaxeman]

The SU 800 speed light commander can control 3 groups, so I guess only being able to use 2 is a limitation of using the camera's inbuilt commander function.