What's the specification of the quaternion protocol?

[tribbloid]

The documentation is very blurry on this part so I'd like to suggest to explicitly defined it in the code, presumably with the following information:

• neutral heading / zero heading of the glasses: where does the user look into when all reading are 0?
• reference frame: should be glasses body frame or one of the world inertial frames
• chirality: should be left-handed or right handed
• coordinate system: should be XYZW, WXYZ, or some esoteric coordinate system
• marshalling sequence: should be ascending or descending
• normalisation: should be normalised or unnormalised

Euler angle protocol has similar specification but I was able to reverse-engineer most of it from the Unity example

[tribbloid]

If I switch to quaternion API in Unity, the result also becomes quite unstable

on 90 degree roll the error between quaternion & euler angle may reach over 50 degree

should we just stick to Euler angle?