Dose-response curve
A plot of how a response changes across a range of an input. For CO2 it means photosynthesis measured at a series of concentrations, which is the only way to locate a saturation point.
The value of a dose-response curve is that it covers a range instead of comparing two points. Two treatments can show that more CO2 gave more photosynthesis. Only a curve shows where the gain stops being worth the gas.
The shape that matters for enrichment is saturation: a rise that flattens as the input stops being the limiting factor. The concentration where it flattens is the economically useful setpoint, and it can only be identified if the tested range extends past it.
A curve is evidence only across the range it was measured. Continuing the trend beyond the highest tested dose is extrapolation, and that is where most confident claims about high CO2 setpoints in cannabis come from.
The published cannabis work tops out at 750 ppm with assimilation still rising, so no saturation point has been established for the crop. Setpoints above that range rest on general C3 physiology, on the cost of gas, and on worker safety.