For the gauge, the color-coding relates to research-paper averaged weekly age/gender specific HRV stats for low, mid, high. Lower is “fight or flight – Sympathetic” and higher is “rest and digest – Parasympathetic”.
As a rule of thumb, HRV should drop if you are under load and the body is responding to stimulus (work, gym, fasting). As the stimulus declines, HRV should recover. Avoiding extra stimulus of the same type when HRV is down is a technique to avoid injury. There’s no such thing as a good HRV or bad HRV, but an HRV which is trending higher over time is strongly associated with longevity.
More advanced wearable usage layers additional information e.g. a rugby player might choose kicking rather than tackling practice if HRV is down and Readiness is not. Other data might be layered for training decisions including V02Max and DC. Personal Trainers working with top athletes observe that an innate ability to adjust output to circumstances separates the top athletes from those who might otherwise be able to operate at the same level. The combination of the right Personal Trainer and wearables data creates an opportunity to level the playing field and climb a rung on the performance ladder while avoiding injury.