Nikon D6 Reference Manual page 1003

D
Number of Shots
[No. of shots] shows the number of times the shutter has been released.
Note that the camera may sometimes release the shutter without
recording a photograph, for example when measuring preset white
balance.
D
Charging Batteries at Low Temperatures
Batteries in general exhibit a drop in capacity at low ambient
temperatures. Even fresh batteries charged at temperatures under about
5 °C (41 °F) may show a temporary increase from "0" to "1" in the value
shown for [Battery age], but the display will return to normal once the
battery has been recharged at a temperature of about 20 °C (68 °F) or
higher.
Slot Empty Release Lock
Choose whether the shutter can be released when no memory
card is inserted in the camera.
Option
[Release
a
locked]
[Enable
b
release]
The shutter cannot be released when no memory
card is inserted.
The shutter can be released with no memory card
inserted. No pictures will be recorded; during
playback, the camera displays [Demo].
G button U B setup menu
Description
B The Setup Menu: Camera Setup 951