Nikon D6 Reference Manual page 1036

D
[Add]
•
Colors and brightness in the preview may differ from the final picture.
•
The overlay will include only NEF (RAW) pictures created with this
camera. Pictures created with other cameras or in other formats cannot
be selected.
• The overlay is saved at an [Image quality] of [JPEG fine c ].
•
NEF (RAW) pictures can be combined only if they were created with the
same options selected for:
- [Image area] > [Choose image area],
- [Image size] > [NEF (RAW)], and
- [NEF (RAW) recording] > [NEF (RAW) bit depth]
•
The overlay has the same photo info (including date of recording,
metering, shutter speed, aperture, exposure mode, exposure
compensation, focal length, and image orientation) and values for white
balance and Picture Control as the photograph selected for [Image 1].
Copyright information, however, is not copied to the new image. The
comment is similarly not copied; instead, the comment currently active
on the camera, if any, is appended.
984 N The Retouch Menu: Creating Retouched Copies