it's an indicator for offset calibration status for each sensors (accelerometer, gyroscope and magnetometer) as defined by Android. The calibrations is reported for each sensors by 2 bits, value of 3 indicated sensor is fully calibrated and 0 indicates that the sensors is not calibrared. in addition there a calibration status for the full system, which depends on status of all 3 sensors. For more information please refer to BNO055 quick start guide