Nikon D3 - Manual (Page 136)

110
S
Auto ISO Sensitivity Control
If [Off ] (the default setting) is chosen for the [ISO sensitivity auto
control] option in the shooting menu, ISO sensitivity will remain
fixed at the value selected by the user (see page 108). If [On] is
chosen, ISO sensitivity will automatically be adjusted if optimal
exposure can not be achieved at the value selected by the user
(flash level is adjusted appropriately). The maximum value for
auto ISO sensitivity can be selected using the [Maximum
sensitivity] option in the [ISO sensitivity auto control] menu (the
minimum value for auto ISO sensitivity is automatically set to ISO
200). In exposure modes e and g, sensitivity will only be adjusted
if underexposure would result at the shutter speed selected for
[Minimum shutter speed]. Slower shutter speeds may be used if
optimum exposure can not be achieved at the ISO sensitivity value
selected for [Maximum sensitivity].
When [On] is selected, the viewfinder and
rear control panel show ISO-AUTO. When
sensitivity is altered from the value selected
by the user, these indicators blink and the
altered value is shown in the viewfinder.