Sony FX 6 Quick Start Manual page 56

Exposure: Base ISO/Sensitivity
As explained in the section on Custom mode settings, the FX 6 camera is a
dual sensitivity camera so it performs optimally at two distinct Low and High
ISO settings. These settings change with the Shooting Mode. The ISO settings
for Custom Shooting mode are in bold, below:
Base Low:
Custom (709) Low: 320 ISO
Cine EI Low:
Base High:
Custom (709) High: 5000 ISO
Cine EI High:
800 ISO
12800 ISO