Quote - "... the difference between "pure layers" and "layers and mixed materials ..."

If you notice in my material, I have all 4 materials in a "pile". One of the materials is the eco, of course, and it's sitting on top of the 2nd material, so its distribution is restricted to that material. The materials in this "pile" are distributed on the terrain based on altitude (see the slider "altitude range"). To create materials like this, you just click on "add layer", one after the other.

In your case, you have a hierarchy, you have a "tree" of materials. This indicates mix materials. The two in the bottom are being mixed. The one on the top could have been added as a layer (before you mix the two bottom ones, otherwise Vue wouldn't let you) or it could also be mixed with the result of the bottom mix (I can't tell from the image). In any case, it's substantially different from what Peggy explained she was doing and very different from what I did (which was based on Peggy's description).