Nikon Z 6II Reference Manual page 656

D
Shutter Speed
Shutter speed can be set as follows when an optional flash unit is used:
Mode
b
P, A
S
M
* Shutter speed may be set as slow as 30 s if slow sync, slow rear-curtain
sync, or slow sync with red-eye reduction is selected for flash mode.
D
Third-Party Flash Units
The camera cannot be used with flash units that would apply voltages
over 250 V to the camera's X contacts or short-circuit contacts on the
accessory shoe. Use of such flash units could not only interfere with
normal operation of the camera but also damage the flash sync circuits of
the camera and/or flash.
Using an On-Camera Flash
614
Shutter speed
Set automatically by camera (
Set automatically by camera (
1
1 / s–30 s, Bulb, Time
200
1 /
200
1
/
200
/ s–30 s
200
s– 1 / s)
60
1 *
s– / s)
60