Nikon Z 8 Reference Manual page 276

D
Cautions: "Lighten" and "Darken"
Only pictures created with this camera can be selected. Pictures created with other models cannot
be selected.
The overlay will include only pictures created with the same options selected for [Image area] >
[Choose image area] in the photo shooting menu.
The image quality setting for the completed overlay is that of the highest quality picture it contains.
Overlays that include NEF (RAW) pictures will be saved at an image quality of [JPEG finem].
All JPEG pictures in the overlay must be the same size.
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"Lighten" and "Darken"