I also don’t understand quite well how this data works, but "z" is the 3d plane, where you get the distance from kinect. I think the floats from tracking are relative to each inicial point tracked in OSCeleton, not regarding the distance/space in itself.
Anyway, I wrote a set of abstractions called KOSCeleton that you can put it in your abstractions folder, so you can use in your patch only the point you’re interested in. To do so, just add the letter k, and type the name of the point as you’d do with a regular max object, for example: khead, kl_hand, and so on. The outlets gives you x, y and z. By doing this you avoid always having to work with that huge patch that tracks the whole body.