Heart rate zones and max heart rate
Your maximum heart rate is used to work out your heart rate zones, which color your heart rate on the watch and make zone targets possible in interval workouts.
Max heart rate
On the phone, Settings › Profile › Max HR. If you don’t know it, Estimate works it out from your age (220 minus your age). That formula is a rough guide with a wide margin of error from person to person, around 10 bpm either way; if you have a figure from a lab test or a very hard race, enter it yourself instead.
The zones
There are five zones. By default they’re worked out as percentages of your max heart rate; with Customize you can enter the lower limit of each one yourself (for example, if a coach or a lactate test gave them to you). Without a max heart rate there are no zones: heart rate shows without a zone color, the arc on the watch is a red bar without zones, and zone targets aren’t judged and don’t alert you.