Regulation of line weight

Dear all, 

Could someone help to show me the regulation relating to line weight in Microstation and OpenRoad Designer?

Differ with Autocad, line weight in Microstation showing number 0, 1, 2, 3, ... and I don't know the correct thickness.

For example, line weight 1=?mm, ...., and which regulation for this in USA or Euro?

Thank you in advance