Pen thickness is the display and printing width for entities. Usually, Line Width is the best option.
A line with a pen thickness of 0.5mm will print out 0.5mm wide, regardless of the zoom level. A wall with a line width of 1 foot will appear at a different width, according to the zoom level.
Specify or change the desired pen width by clicking directly on the indicator. CC3 displays the Pen Thickness dialog, which lets you specify paper scale thickness and mm/inch units. It is independent of line width.
You can use pen thickness to increase the display width of zero-width entities when you print or export to bitmap or PNG.