Each zone has a specific benefit that comes from the physiological activities that happen when you exercise within that heart rate zone. For example, the 21 Healthy Heart zone is exactly that, the range of heart rates where most individuals realize the most cardiovascular benefits, leading to improved heart and lung function. Zones have numbers. There are certain specific and measurable events that are so exact that they're represented by a single heartbeat value called a heart rate numbers. For example, the diagram below is the location of your Max HR on the ceiling of Zone 5.
Zones are a subset of the wellness continuum.
The health area covers those training zones that promote health but don't primarily improve physical fitness and certainly not performance. To measure improvements in health, we seek positive changes in blood pressure, body fat, cholesterol, etc. To measure improvements in fitness, however, it's positive changes in oxygen utilization, lactate concentrations and heart rate points we're looking for. And, to measure improvements in performance, positive changes in completion times, accuracy of movement skill, mental attitude, and other indices are used. Zones use time, not distance, as their measurement tool.