:msp_thumbup:
Shigo used "barrier zone" and "wall 4" interchangeably in his work. Many think of the "barrier zone" as being existing wood chemically altered to resist decay and "wall 4" being a physical anotomical barrier to decay layed down over the wound in subsequent growth increments. No callus/wound wood then no wall 4 or closure = wall 4. Some may believe a mix of the two or none of the above. All should aim for proper pruning cuts and let the tree sweat the myriad details of when and where it compartmentalizes.
So, my jest earlier aimed @ Guy was a wink and a nod toward the likelihood that half the participants in this thread may believe wall 4 to be a chemical barrier zone inside the tree @ time of wounding and the other half may believe wall 4 to be located inside the callus/wound wood material as it grows over the wound.
I'm comfortable with my own understandings but many arborist remain confused about this aspect of the CODIT model.