How the Reference System in Unreal Engine Is defined?

I have some problema in understanding the Engine Coordinate System.
I am using a HTC VIVE pro set for virtual reality. So i have defined the room using STEAM VR.
For what i understand:

  • the Z axis Is directed Howard
  • the X axis Is pointed in the forward direction defined using STEAM VR
  • the Y axis Is defined using a Left Handed Convention.

Now the problem Is that if I move the HMD in the space I obtain strange values of Roll Pitch and Yaw.
Specifically the Yaw respond correctly to the Left Handed System, however both Roll and Pitch respond to a Eight Handed Reference with positive anti-clockwise rotations.

Can someone explain me how the Reference System works?