But, as you move up to higher intensity zones, you need to decrease the amount of time that you spend in that zone, particularly in the top two, the Anaerobic and Redline zones. Zones are relative. Your five heart rate zones are specific to your maximum heart rate, not anybody else's. With two runners, each maintaining a heart rate of 160 bpm, one might well be in their 24 Threshold Zone and the other may be in their 22 Temperate Zone. It's all relative.
Zones are metabolic, calorie burning zones.
Each heart zone burns a different number of calories per minute based on how fit you are. Fat is burned differently in each of the heart zones. You'll burn a different ratio of fat to carbohydrates in each of the heart zones. That's because oxygen has to be present for fat to burn. If there's no additional oxygen present, there's no additional fat burned.