While the overall benefits of cardio are vast, the primary purpose in fitness programming is to reduce body fat. Most people think that you can drop body fat by only working out for an extended period of time on a certain machine or in a class. Most people also believe that there is a "fat burning" zone for exercise. However, body fat reduction can only take place when there is a caloric deficit. When there is more energy being burned than consumed. This is the Law of Thermodynamics.
Typically, the "fat burning" zone is thought of as when the body is mainly using fat for fuel. Fat and glucose are major sources of fuel for exercise. In order for them to be used efficiently, the body must receive enough oxygen. Oxygen allows fat and glucose to be used as fuel, or energy. This in turn produces carbon dioxide and water as waste products. By using a metabolic analyzer you can figure out a person's respiratory exchange ratio (RER), which will measure how many calories are being burned in a specific exercise.
The body uses the highest percent of it's fuel from fat when the body has an RER of 0.71. So how do you exercise at this level? You don't. The body can only be at 0.71 RER when is at complete rest. This is how the fat burning zone started.
Thought the percentage of fat being burned is at the max, the amount of energy used and calories burned is minimal. This isn't very productive in achieving weight loss or body fat reduction. So, it's not how much fat an individual burns, but how many calories that ultimately produces results.