Scratch

I think he's suggesting a third spatial axis to let him make true 3D projects. But it won't do to just introduce a new pseudo variable...

There has been discussion of z-axis for pseduo-3D on other threads. 

My son and I discussed how to add a proper z-depth system compatible with the existing rendering and commands.
  We came up with two new primitives:
      1) a z-depth variable that can be set or read like other state variables.
      2) a set-all-z to layer
   
  The idea is that each sprite has a Z value.  To do rendering, the
  z-values are sorted with a stable sort to get the same layer list as
  is currently used.  If one stays entirely in the set/read Z model,
  this is the only change needed.
 
  To maintain compatibility for existing programs, the meanings of
  move to front and move back n layers needs to change slightly.
  The operations are done as now, but then the Z value is set to the
  layer number (layer 0 or 1 is the closest, and stage is at depth infinity).
 
  Users can get access to the set-all-z-to layer function by doing
  "move back 0 layers", so only the z-depth variable needs to be added.

I'd be glad to explain it again, but I'm not sure what parts you didn't understand.
Are you familiar with the (x,y,z) notation for three dimensions?
Do you know how Scratch currently handles one sprite being in front of another?

Once you add 3d to a game or project, it will make it allot more complex to make a project or game.  You would have to be able to make 3d spites too.

The suggestion that my son and I had was not adding true 3D---that would raise the complexity a lot.  Instead, it was offering better control of the layering of the 2D sprites (sometimes called 2.5D).

The sprite will *always* have a position in the rendering list.  The suggestion that we made allows all the current operations to work, but also allows programs access to the information about what sprite is where on the list.