Specifying the orientation property of a conr with a quaternion

CesiumJS
1

The orientation of a cone can be set in cesium with a quaternion, from the property "orientation" of the entity. Does anyone know what the quaternion is defined respect to ?

2

Have you seen my answer to this question in your other thread?

https://groups.google.com/d/msg/cesium-dev/tcF5ZiPwbY0/CO8FURaWAAAJ