This is copied from the Zf reference guide, but I'm sure the Z6III is the same.
To enable subject detection during manual focus, select an option
other than [MF subject detection off] for [MF subject detection
area] in the photo shooting or video recording menu ( 0 128).
If [Auto] is selected, the camera will detect humans, animals, and vehicles and choose a subject for
focus automatically.
To disable subject detection altogether, choose [Subject detection off].
Using Subject Detection During Autofocus
Subject detection is available during autofocus when [Wide-area AF (S)], [Wide-area AF (L)],
[Wide-area AF (C1)], [Wide-area AF (C2)], [3D-tracking], [Subject-tracking AF], or [Auto-area
AF] is selected for [AF-area mode].
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