Let's say we walk on a treadmill for 30 minutes at a respiratory quotient of 0.8 (RQ=The volume of CO2 output divided by O2 intake over a period of time. More than you need to know, but there it is so you don't have to look it up. I know you really wanted to. :-) ) and it burns us 110 calories. At this RQ rate we should be burning about 65% of our calories from fat. So, we burned off roughly 71.5 calories in that half hour. Yay for us, there goes the four whole chocolate covered pretzels we ate earlier (doesn't seem worth it, does it?).
Now, let's keep the timing the same, but bump the intensity up to a moderate run so we get the RQ closer to 0.9 (the RQ scale runs from 0.7-1.0 for reference). With this increase effort, we're only burning about 32% of our calories from fat. So, this sounds like a bad place to be doesn't it? Why would we want to put forth more effort for a smaller percentage of payout? Well, we're not actually because at this higher intensity we're burning more calories overall. See where this is going? At this higher intensity, we might be burning about 260 calories total which means we're up to around 83.2 calories from fat even though we are at a lower percentage of fat burned. The difference between the two amounts of fat calories here and in the paragraph above may not seem like a lot, but remember that exercise is cumulative and every little bit counts over time. Plus there's a bigger difference in the calorie count overall. This isn't to say you should hit it hard every time; we still need to balance out with lower intensity training here and there. It is to say, "Don't be afraid to kick it up a bit."
So, that's it: With just a few paragraphs and a bit of math, the "Fat Burning Zone" myth is busted. It's simply a tool used by marketers of exercise equipment, fitness trackers, etc. to entice you into buying one particular model over another. But now that you know their game, you're one step ahead of them. So, if the sales person tells you the device has a "Fat Burning Zone", just give them a vacuous stare, smile, and move on with your next question. The math doesn't lie.
I hope this has shed some light on at least one crazy aspect of the world of fitness and that you'll use this information the next time you exercise (Which should be right now. Go! Get another 15 minutes in! Seriously, I'm not kidding. Go!).