There are some special characteristics of the five heart zones that make them what they are. The key fact is that a different thing happens in each of the different heart zones. This means that you have to train in different zones to get each of the different benefits. 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.