What is the myth about the fat burning zone?

i think it occurs when you do anaerobic exercise, you can see that with your heart beat, it should be in a specific zone, most usual is by subtarcting to the maximum heart rate your age. for instance, if you are 20 years old and your maximum heart rate is 200 your heart rate should be around 180 bpm, if doing exercise and keeping this heart rate, you should be burning some fat.