This is a 4mm thick hex nut calibration plate, helping you size hex nuts, shafts and other hexagonally-shaped fasteners with a tolerance of ±1mm either way. It covers common metric sizes: M3, M4, M5, M6, M8 and M10.

The millimeter measurement on the left refers to distance between the flats. This means you need to adjust it for your CAD system. For example, if you are using OpenSCAD, to get the radius of a cylinder (with $fn=6), you take this value and divide it by sqrt(3).