Nikon Z 8 Reference Manual page 120

Focus indicator
(steady)
(flashes)
When using manual focus with subjects not suited to autofocus, note that the in-focus indicator (I)
may be displayed when the subject is not in focus. Zoom in on the view through the lens and check
focus. Use of a tripod is recommended when the camera is having trouble focusing.
D
Lenses with Focus-Mode Selection
Manual focus mode can be chosen using focus-mode selection controls on the lens (where available).
D
The Focal Plane Mark and Flange-Back Distance
Focus distance is measured from the focal plane mark (E) on the
camera body, which shows the position of the focal plane inside
the camera (q). Use this mark when measuring the distance to
your subject for manual focus or macro photography. The distance
between the focal plane and the lens mounting flange is known
as the "flange-back distance" (w). On this camera, the flange-back
distance is 16 mm (0.63 in.).
Tip: Focus Peaking
If [ON] is selected for Custom Setting a13 [Focus peaking] >
[Focus peaking display], objects that are in focus will be
indicated by colored outlines that appear when focus is adjusted
manually (focus peaking).
Note that the focus peaking display may not appear if the
camera is unable to detect outlines. Check focus in the shooting
display.
Focus is behind the subject.
The camera is unable to focus.
120
Description
Focus