The Heart Rate screen is used when you pair your device with a Heart Rate Monitor (HRM). I don’t know how accurate the calories are since I feel that they are really only an estimate on any device (including when I use my paired HRM), so I just use that as a rough estimate.
https://www.amazon.com/Garmin-v%C3%ADvofit-Activity-Tracker-Black/dp/B00RY1Z9NQ

