Few questions about the radar point clouds

[amirbakhtiary23]

Hello,
Thank you for your great contribution. I had a few questions about radar point cloud and didn't want to bother you with email.
According to the paper :
The 3+1D radar outputs a point cloud with spatial, Doppler and reflectivity channels for each scan, giving a total of five features for each point: r range, α azimuth, θ elevation, vrel relative radial velocity, and RCS reflectivity. Since most point cloud based object detectors use Cartesian coordinates, we also transform the radar point cloud: p = [x, y, z, vrel, RCS], where p denotes a point, and x, y, z are the three spatial coordinates with x and y axes pointing forward and left respectively w.r.t. the vehicle, see Figure 2. Compensated radial velocity is a signed scalar value denoted by vr, describing the ego-motion compensated (i.e. absolute) radial velocity of the point.
Now the questions, if we consider these x,y,z as coordinates, which one would represent the depth? I suppose its x.
And about the y,z. Since it's mentioned that Z represent elevation, i suppose it means the height, but how is height defined in these cases. I would appreciate if you could help me. Sorry if my questions are basic, I'm a CS major and don't know much about radars.