Zones have size. The size of the zone in number of beats depends on how high your true Max HR is. Zones have structure. In other words, inside every zone is an upper and and a lower zone. It's just a way to subdivide a medium-sized window into two smaller, even more focused parts.
Zones have dividing lines.
The upper and lower limits of each zone coincide with the floor and ceiling of its bordering zones. This floor, or lower limit is that heart rate where you first break into this zone. Seventy percent of your Max HR also happens to be where the Fat Burning zone ends. At this point you are passing through the Aerobic zone ceiling into the floor of the next higher and more intense zone, the Anaerobic zone. Zone names correspond with their benefits.