Answer
9 axis MotionTrackingĀ® combines gyroscope, accelerometer, and TMR magnetometer sensor data. The resulting angular frame is relative to both gravity (down) and north.
The advantages of 9 axis MotionTrackingĀ® over 6 axis MotionTrackingĀ® are that 9 axis MotionTrackingĀ® results are usable for navigation and other applications which require information about the angle relative to north. It also eliminates any heading drift which may be present in a 6 axis MotionTrackingĀ® solution so that users are pointed in the correct direction without delay due to recalibration.