The area put forward for certification has to be a space that can be closed off completely by its walls, floors, ceilings, doors and windows, and every door and window can be locked or secured in some other way. The guideline explains that together these surfaces are the containment barrier that protects the rest of the building and the outside environment.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.