What is the difference between cAD global width and line width? What effect does it have on printing? How to set it to achieve the printing effect?

Simply put, the default. The line width is not displayed in the model space, but you can choose whether to display the width when printing. Whether in model space or in printing, the global line width will be displayed; The former is linear display and the latter is geometric display. For example, this set of pictures:

If you want to display the line width directly, you can select it in the line width setting: