Sony FX 6 Quick Start Manual page 74

Exposure: ND Auto
The best way to maintain a consistent manual iris setting is to use ND Auto.
Keep the iris on Manual.
Press the ND ON button, switch it to ND Variable and then hold down the AUTO ND
button until AUTO ND appears in the LCD display. A light goes on the button as
well.
Now the camera will vary the strength of the ND filter automatically to maintain
your chosen iris setting (providing of course that your iris setting is overexposing
the image). In AUTO ND, the lowest option is ¼. It does not go to "clear". You
always have some amount of ND filter applied.
The display will warn you if the light is low (meaning that you are underexposing by
having the ¼ ND applied).