If you have recorded activities with heart rate data, you can also let it display the heart rate zones.
The definition of the heart rate zones is not standardized. Usually there are five zones and they have the same names. What differs is how their ranges are computed and there is some chaos around that.
All definitions that I found take the maximum heart rate as the upper limit. One can measure this as part of a professional training or just use the 220 minus age prescription which at least for me matches close enough. What they differ on is how they use a lower bound. It seems that Polar or REI basically use 0 as the lower bound. My Garmin system also uses 0 as the lower bound. But as one can see in this blog, one can also use the resting heart rate as the lower bound.
Based on the maximum and resting heart rate we will then compute the heart rate zones using certain percentages of effort. We can compute the heart rate as the following:
rate = effort × (maximum – minimum) + minimum