Nikon D6 Reference Manual page 1037

Lighten and Darken
The camera compares multiple selected pictures and selects
only the brightest or darkest pixels at each point in the image to
create a single new JPEG copy.
1
Select [Lighten] or [Darken].
After selecting [Image overlay] in the retouch menu,
highlight either [Lighten] or [Darken] and press 2 .
•
Lighten: The camera compares the pixels in each picture
and uses only the brightest.
•
Darken: The camera compares the pixels in each picture
and uses only the darkest.
N The Retouch Menu: Creating Retouched Copies 985