Nikon D6 Reference Manual page 1074

1 Displayed by pressing button to which [Viewfinder virtual horizon]
has been assigned using Custom Setting f3 ([Custom controls];
0 850).
2 Functions as a pitch indicator when the camera is rotated to take
pictures in "tall" (portrait) orientation.
3 Displayed when [On] is selected for Custom Setting d11 [Framing
grid display].
4 Displayed only when a lens with a built-in teleconverter is attached
and the teleconverter is in use.
5 Functions as a roll indicator when the camera is rotated to take
pictures in "tall" (portrait) orientation.
1022 Camera Displays