Sony FX 6 Quick Start Manual page 36

Audio: Low Cut option
There is a switch on the microphone for a low frequency cut to the
signal. This is not necessary unless there is a persistent low hum in the
environment that you want to remove.
M is no cut.
V is the low cut.
I could not find any documentation on where
the low cut starts. Probably 90 Hz. Otherwise
the microphone can pick up sounds as low as
40 Hz.