Nikon D810 - Manual (Page 422)

398
D
Image Overlay
Only NEF (RAW) photographs with the same image area and bit depth
can be combined.
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. The current image comment is appended to the overlay
when it is saved; copyright information, however, is not copied.
Overlays saved in NEF (RAW) format use the compression selected for
NEF (RAW) compression in the NEF (RAW) recording menu and have
the same bit depth as the original images; JPEG overlays are saved
using size-priority compression.