The "Fat Zone" or "Fat Burning Zone" refers to the intensity of aerobic exercise necessary to maximally burn body fat without burning-up the liver's store of carbohydrate (Called glycogen).
In order to use fat as a fuel for exercise, we have to keep it slow. Above a certain level of intensity, roughly 65-75% of maximum oxygen consumption, fat won't work and we begin burning the liver's store of carbohydrate called glycogen.
As stated above, we stop burning fat at about this point. But what is VO2MAX?
VO2MAX and heartrate are generally about proportional and better still, related to each other in a linear way. This means that you don't need an exercise physiology lab to know when you are in the fat burning zone. You just need to check you pulse. If you are below about 65-75% of your maximum heart rate, then you are burning fat and not glycogen.
Here is a link to our Holland Clinic Fat Zone Calculator