This difficulty unlocks once you complete the game. It is a no death mode. If you die, your save file can't be used. You can reload though prior to a death. So if you mess up and get seen, you can reload your last save and try again. There is no auto save. I would recommend saving often. If you want, you can back the save up to USB or the cloud. If you die, you can overwrite it from the back up and try again.
I take no credit for this video. I found it when I was going for this and it is incredibly helpful. I bought what they bought, looted what they looted etc. As I didn't need to speed run, I also did a bit of searching in some areas so I ended up with extra biocells and praxis kits which helped a little more. I also got Foxiest of the hounds using this video: