To the point of this thread, the Polar V650 shows what zone you are in while riding. In the graphic below is shows the rider is in zone 3 and has been there for a total of 32 min 15 sec.
If the heart rate display is tapped, it shows a detailed view of that zone, where you are within the zone and beeps when you go above or below the zone. This is called "Zone Lock."
I find both zone displays incredibly useful for metering effort, especially at the end of long, hard rides. The first graphic at the top of this post is my primary metric, on screen almost all the time.
This is the sort of functionality I'm looking to replicate but understand the way it is displayed to the rider may be different across brands. Any further information about how this is implemented would be welcomed.
-Tim-