Sony FX 6 Quick Start Manual page 17

Recording Media: Proxy recording and Simultaneous Record
Proxy Recording:
Proxy recording creates compressed .mp4 duplicate video files while you record.
I don't recommend it. The proxy files are not useful in an online/offline workflow
as they do not have the identical file names of the full resolution files. I
recommend creating offline files using Adobe Media Encoder and the Pro Res
422 Proxy codec.
You can turn on proxy recording in the status pages and menu if you wish.
Simultaneous Record:
If you want to have copies of the video files recording to each card
simultaneously, one copy per card, turn on simultaneous record:
Menu/Project/Simul Rec/Setting/ON