The reference method relies on an instrument that counts particles one by one and can show or log both the count and the size, with discrimination fine enough to capture the total concentration in each size band relevant to the class concerned. The counter must carry a calibration certificate that is in date; how often and how it is calibrated is recommended to follow accepted current practice as laid down in ISO 21501-4, and if a counter cannot be calibrated to all of that standard's tests, the choice to use it anyway is written into the test report.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.