Yes, in theory if you get above that rate, you’re conditioning as opposed to burning fat, and if you get below it, you’re not burning fat. IIRC, you start burning other tissue above that zone, and if you get into the red zone, you’re “anaerobic,” meaning you’re breaking down tissue wholesale.
I think, however, that you’re still burning fat in those upper zones. It always seemed to work that way for me. You just won’t bulk up—at least in the muscle groups you’re taxing at the time. You can see how cyclists, for example, want to mix in both types of rides. If they don’t get the heart rate down on some rides, they won’t build their quads, etc.
I must have managed to work in plenty of each before I paid much attention. I built some pretty huge quads back when I used to ride around 65 miles (on a mountain bike) every Saturday. I never put on a lot of upper body weight, and when I did push it, my rides did get harder. Don’t think I ever got under 10% body fat.
Since I mentioned cycling, that’s a great solo sport in a warm climate. Hell, here in Colorado, the insane hardcore cyclists attack the mountain passes year-round! I always found running hard on my body, but I could hit the bike trail hard for four-to-six hours.