The room put forward for certification has to be a space that can be closed off completely, bounded by its walls, doors, windows, floors and ceilings, and each of its doors and windows has to lock or be securable by some other means. The guideline notes that these surfaces form the containment barrier protecting every space outside the facility, including other parts of the building it sits in, and the environment.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.