Line widths are paper scale
Check this option to plot or print line widths in scale to the paper (real world), not to the drawing. For example, if you were printing a line with length = 4", line width = 0.25", at a scale factor of 1:2 and with this option checked, the line would print with length=2", line width = 0.25". Leaving the option unchecked would cause the line width to geometrically scale. In our example, with the option unchecked, line length = 2", line width = 0.125".