Yes, the line is grey AFAIK (not a lawyer), and usually is drawn at characteristics related to "commercial exploitation" (which might not even mean taking money

Yes, the line is grey AFAIK (not a lawyer), and usually is drawn at characteristics related to "commercial exploitation" (which might not even mean taking money, but competing with companies that do). It's a similar thing with patents. No one can really forbid (or enforce a ban) you from completely independently coming up with the same idea and executing on it in your garage for personal purposes. However if you then try to commercialize the same idea, you have to face the reality of the world of patents.