Described in the explainer IDL: https://github.com/immersive-web/webxr/blob/master/explainer.md#appendix-b-proposed-idl

The XRPose interface is pretty simple, but needs to be implemented in order for several other related changes to land. The existing XRViewerPose needs to be updated to be extended from XRPose, which simplifies that interface a bit as well. (It now only has to track the views array.)