This is a EXCELLENT question, that probably has more than one answer...so I'll just give you my take...
The term "Natural" is most likely used in order to garner acceptance, both by the general public, and on an international level (e.g.the IOC).
The International push is pretty self- explanatory (although hypocritical when you see how the IOC will often turn a "blind eye" to a lot of abuses...but that's another thread completely!).
On the general public level, there are literally thousands (if not millions) of people willing to take their bodies "to a higher level" but NOT at the expense of their health or by breaking the law. They simply want to push the iron, eat clean, take supplements, and compete to whatever level their bodies will take them WITHOUT breaking laws or by what I call "Hormonal Manipulation"...and when they compete, they want to know that they will be on a "level playing field".
These people are a HUGE untapped market (and Bill Phillips knew it VERY well)...so putting "Natural" on your Federation and/or contest really taps into that market.
On to your question!
What is the "divide" for "Natural" FOR ME? (This is just MY take...I don't speak for anyone else...)...it has to do with what I call "Hormonal Manipulation"...if I take an exogenous substance that suppresses the very natural balance and "rhythm" of our hormonal system (that has derived that delicate balance over thousands of years) because I can get more of it in my body...then I have to take OTHER substances to counteract the side effects...THEN wait and hope that natural hormonal production will return...TO ME, I've crossed the line from being "natural".
This is NOT a "moral" judgment on who chooses to do so...I think that it's just a dangerous game of Roulette that I'm not willing to play...
And for those who support exogenous Hormonal Manipulation by saying that " takin' protein powders ain't natural EITHER, man....!...", it's a ludicrous argument. By taking a protein powder OR by eating a lot of protein, I'm providing raw material (substrate, if you may) so that the machinery of my body does its job more efficiently.