This is what X and Y is ; )
An example:
@CHAR spot 0.900 50 90 in zone 1 at layer 1
layer can be any number= -1, 1, 2, 3, etc.
If you want a character behind a desk, then the character is at a lower layer. So if you put the desk at layer 2, then the character would need to be at layer 1, making them behind the desk. However, if you want the character in front of the desk (and you have the desk at layer 2), then the character would need to be at layer 3, making them in front of the desk.
More on layers here: HOW TO: Move Characters / Overlays to the Layers
Good luck with everything!