Nikon D6 Reference Manual page 704

1 Y icon flashes if flash unit does not support red-eye reduction.
2 Not available in modes S and M. Fill flash is selected instead.
3 Not available in modes S and M. Red-eye reduction is selected instead.
4 In modes P and A, slow rear-curtain sync will be selected as
shown when the W ( M ) button is released.
652
Flash Modes