If you are in heart rate mode, the amount of white fill in the heart rate icon relates to the confidence the algorithm has that it is reading the right value.
The positioning of the heart-rate sensor can govern the accuracy of the measurement results. The wristband technology is designed specifically for the wrist of the wearer. The ideal location of the sensor is approximately 3 finger-lengths proximal to the elbow on top of the wrist.
Ensure the sensor face (on the bottom of the sensor “medallion”, in the picture above) is lying directly against the skin with no gap. Also, for best results, tighten the strap such that the sensor module is stable and does not slide on the skin but also not too tight to constrict blood circulation. Less motion of the device relative to your wrist means a more accurate sensor.
The sensor will still perform when worn in positions near the recommended position (image above), however, the closer to the recommended position, the more likely the sensor is to perform accurately.
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