Nikon D6 Reference Manual page 478

1
Select the desired picture and press the i button.
2
Highlight [Select to send to
smart device], [Select to send
to computer], or [Select to send
(wired LAN/WT)] and press J .
Pictures destined for a smart device
are marked with a W icon, while
those destined for a computer or ftp
server are marked with u .
D
Removing Upload Marking
To remove upload marking, repeat Steps 1 and 2.
426
Selecting Pictures for Upload