The work area has to be a space that can be closed off completely, with walls, floors, ceilings, doors and windows as its boundary, and every facility door and window must be able to be locked or secured in some other way. The guideline notes that these surfaces form the physical containment barrier around the GMO dealings, protecting every space outside the facility (including the rest 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.