Reference frame for linear and angular velocity

Hi all,

I've started for some weeks to use the PC SDK to build a custom application. I navigated through the documentation to find an answer to my question but I couldn't find what I needed so far.

Basically, I see that the function ovr_GetTrackingState returns a class Posef containing all the information about the current state of the display, in particular the linear and angular velocity.

I easily imagine that the position and the orientation returned by the function are referred to the reference frame attached to the positional tracking system, but what about the velocities? Are they expressed in the tracker or in the HMD frame?

Thanks in advance and, again, forgive me if the question is trivial and already discussed.