Describes how far an operator can step in to prevent harm or hazard. The point is to observe the state and move the system from an unsafe state to a safe one. It is more fundamental than ordinary operability because its purpose is harm prevention, not ease of use. The second edition places it under safety, which is a better fit than the first edition's placement under usability.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.