Quote - Could you explain that in more detail? What kind of material zones? Zones as those defined by different mixed materials? Zones as in dependent on slope or altitude? What exactly?

Your understanding is correct.  We were using Chipp Walter's (and others') method of coloring one of two Mixed Materials zones on a mountainous terrain red and the other green with an obvious sharp seam between the two.  Then when we converted the red & green materials to "real" materials, we wanted an EcoSystem with trees to only appear on the green (lower) zone.  However, even though we applied the trees to only the green material, they appeared higher up (in the red zone).  At first, we thought it was a bug for just me, but then Peggy tried it and she came up with the same result.

Come to think of it, I made an example for Peggy during the class: