Personally, I wouldn’t put a lot of stock in any of them. Our bodies are really complex and it’s less on and off switches and more dial turning. It’s not like our biochemical physiology knows what the difference between 13h59m and 14h00m is. If you’re interested in stuff like autophagy, that typically means longer fasts from my understanding so I would err towards the one that indicates longer if you’re looking for it to give you a ballpark idea of what states you may be in.
I wondered about this, too. It’s really hard to tell what applies, because based on what you eat and how much you eat, your body enters the actual fasting time earlier or later. The beginning of the fast isn’t necessarily fasting yet, it’s just not eating anymore.