Nikon Z8 User Manual page 186

If an aircraft is detected when [Airplanes] is selected, the focus
point will appear over the aircraft in question. The camera will
detect the body, nose, or cockpit depending on the aircraft's size.
If [Auto] is selected, the camera will detect humans, animals, and vehicles and choose a subject for
focus automatically.
To disable AF subject detection altogether, choose [Subject detection off].
If more than one subject of the selected type is detected, a gray focus point will appear over each
of the subjects detected. If [Auto-area AF] is selected for [AF-area mode], e and f icons will
appear on the focus point selected by the camera. The focus point can be positioned over the other
subjects by pressing 4 or 2.
During playback, you can zoom in on the subject used for focus by pressing J.
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Focus