What does accuracy values between 0 and 3 mean for each output (accelerometer, gyroscope, magnetometer)?
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