Nikon Z 7 Reference Manual page 350

D
Image Overlay
Only large NEF (RAW) photographs with the same image area and bit-
depth can be combined. The overlay is saved in JPEG finem format and
has the same photo info (including date of recording, metering,
shutter speed, aperture, shooting 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.
326 Menu Guide > N The Retouch Menu