Sony FX 6 Quick Start Manual page 57

Exposure: Base ISO/Sensitivity
When shooting in Custom mode, Base Low 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 Base Low or High setting without
GAIN or ISO adjustments.
The FX6 allows you to switch between displaying/adjusting ISO or GAIN
on the L,M and H preset switches on the side of the camera.
Adjusting ISO or GAIN does the same thing. It's much easier to use
GAIN than ISO. See next page.