Hi.
While it is always a good practice to give the wall core the highest priority -so you don't have issues- and work outward from there giving the outer skins less priority, in the case of external finishes, it is advised to have two different plaster materials, giving the outer one a bigger priority -maybe even higher than the core- so that other skins won't cut it.
As for insulation and air-spaces inside walls, while you would think they would need a lesser priority (as in insulation is less strong than concrete, right?) you would want it to have a bigger number so that it is a continuos skin.
Hope that makes sense.
Best regards.