@ingenuityprecision , @ikarus is correct , the max flow test uses normal slicing process, but in the design sets object specific modifiers for width/height and changes the speed every layer , also overwrites maximum volumetric flow to 200mm/3
After you slice, the slicer will calculates out how much extrusion is needed for each segment in the print for that speed and filament specified flow to achieve that width and height , at some point the extruder will just not been able to deliver the requested material no matter how much the extrusion (translated to Z steps) is increased due to the material, resistance ad size of the nozzle at which pint artifacts are seen and that is you maximum flow .
Hope is more clear
The max flow test for 0.8 nozzle uses 0.64mm height and 1.4mm width
for 0.4mm nozzle uses 0.32 height and 0.7mm width