Training Zone, using the Karvonen formula ie relative to the HF dynamic range. This app converts % to zones as follows: 50-60%: Zone 1, Moderate Activity (Maintenance/Warm-up); 60-70%: Zone 2, Weight Control (Fitness/Fat Burn); 70-80%: Zone 3, Aerobic (Cardio Training/Endurance); 80-90%: Zone 4, Anaerobic (Hardcore Training); 90-100%: Zone 5, VO2Max (Maximum Effort). It's (mis)using the App rounding characteristic to drive the zone number directly. Unfortunately less than 50% generates minus zones, couldn't avoid it. Zones over 5 shouldn't come up if your HFMax is set properly, that's unless you're suffering from sudden cardiac arrhythmia. Enjoy.