A 0–5 scale measuring the aerobic impact of your workout, as recorded by your Garmin device:
Your heart rate reserve is the range between resting HR and max HR. HR Reserve % shows what fraction of that range you used. Running at 70% HR Reserve is a common aerobic endurance target.
Training Impulse quantifies load using duration and heart rate zones. Unlike power-based training load, TRIMP works for any activity with heart rate. A higher TRIMP means more physiological stress, regardless of sport type.