Nikon D6 Reference Manual page 476

A
Ratings
Ratings can also be viewed in ViewNX-i and Capture NX-D. Protected
pictures cannot be rated.
A
Rating Pictures Using Camera Controls
If [Rating] is assigned to [Fn button for vertical shooting] or [Fn3
button] using Custom Setting f3 [Custom controls], pictures can be
rated by holding the selected control and rotating the main command
dial.
424
Rating Pictures