Apple Watch Core Motion - Gravity and Quaternion

Is anyone able to answer any of these questions? There's very little information on them on Apple's website and I am struggling to find information about them online in relation to the Apple Watch.

  1. What exactly are gravity and quaternion?
  2. How are they being calculated (I'm assuming some kind of sensor fusion between the accelerometer and gyroscope)?
  3. What are the units of gravity and quaternion?
Apple Watch Core Motion - Gravity and Quaternion
 
 
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