Irrigation and lawn watering systems are classed by type: A (all outlets and pipes more than 150 mm above ground, no ponding, back-pressure or injection, no device needed), B (domestic, lower outlets), C (non-domestic, lower outlets) and D (fertiliser or chemical injection); types B, C and D must be protected against backflow with devices suited to the arrangement (2003 text).
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.