Understanding Eye Tracking Data - what do the x, y, z coordinates refer to?

Hi,

I'm hoping to get some more information on how the eye tracking coordinates work:

For left and right eye, the HP headset can provide:

- [EyeGaze] Gaze: x,y,z eye gaze data

- [PupilPosition] PupilPosition: x,y position data

It can also provide combined eye gaze data:

[EyeGaze] CombinedGaze: x,y,z eye gaze.

I found a blog post describing how the Tobii eye coordinates system works (image below), but I'm not sure if the HP eye tracking system is similar. 

It would be great to get some further information on:

- Why does eye gaze use x, y, z coordinates and pupil position use x, y coordinates? And,

- How is combined gaze calculated?

Regards,

Branko