Nikon D6 Reference Manual page 1120

D
The HDMI/USB Cable Clip
To prevent accidental disconnection, attach the supplied clip to HDMI
cables or to the supplied USB cable as shown (note that the clip may not
fit all third-party HDMI cables). The illustrations show the USB cable.
USB cable HDMI cable HDMI cable and USB
cable used
simultaneously
1068 Other Compatible Accessories