Sony FX 6 Quick Start Manual page 73

Exposure: Variable ND filter adjustment
This is the best way to work with the ND filters.
Sony developed this technology.
Press the ND ON button
Turn the ND switch to ND Variable and then adjust
the dial.
This way you can maintain the manual iris setting
you want while applying incremental amounts of
ND. You can also make ND adjustments while
shooting.
The ND AUTO is even more helpful (next page).
This dial becomes your exposure control.