Sony FX 6 Quick Start Manual page 52

Custom Shooting Mode: Two Base ISO settings
Most video cameras have one optimal ISO setting that corresponds to the
sensitivity of the camera's CMOS imaging sensor. At this setting, the camera
is able to get the most dynamic (luminance) range possible out of the sensor.
The FX 6 camera is a dual sensitivity camera so it performs very well at two
distinct Low and High ISO settings. There is a slight increase in image noise at
the High setting.
When shooting in Custom mode, the Base Low sensitivity is the lowest ISO
possible on the camera. You can only add ND to cut sensitivity. Always try to
start with this ISO setting when shooting outdoors or in bright settings.
Ideally, try to shoot with either the Low or High setting without GAIN or ISO
adjustments. Adjusting ISO or GAIN is the same thing. See the section on
Custom mode: Exposure ISO and GAIN.