The colours in that diagram are most unhelpful...clearly the intention is to show decay behind the white face of the injury...but the difference between the brown and orange is almost impossible for me to detect!
So please disregard my inclusion of the diagram have only succeeded in further muddying the waters there!
My second blue arrow is pointing at what the original author intended to be seen as decay (brown colour)of that I'm pretty sure not barrier zone (orange colour)...sorry everyone for that!!!
Irrespective of the diagram and my tangent, Shigo's description of what he meant by wall 4/barrier zone
is clear.