Sony FX 6 Quick Start Manual page 61

Exposure: ISO Preset settings (really important)
Using ISO is more complicated. Look at these examples for Custom Shooting mode:
When using ISO at the Base Low setting (320 ISO), try not to amplify the signal to more
than double the base ISO: 640. 800 ISO can look OK too.
When using the Base High setting (5000 ISO): try not amplify at all or beyond 10000 ISO.
There is a point where amplifying the Base Low signal (320 ISO) makes the image look
noisier than switching to the Base High setting (5000 ISO) even if you need to add a ND
filter to that Base High exposure.
Changing the ISO to 5000 when in the Base Low setting is NOT the same thing as
switching to the Base High (5000 ISO) setting. Increasing ISO on the Base Low to 2500 ISO
creates much more image noise than switching to the Base High setting even though the
Base High setting is 5000 ISO. The Base High setting looks incredibly good!