FP'ers,

This is a newbie question that probably been answered a zillion time but could find it so don't shoot me over it. I keep reading and watch videos that talking about nib number, as in "this is a #5 nib". I know the last number on the MB Meisterstuck, such as 146 or 149 is the nib number. Can some explain to me what are the difference with these nib numbers?

Thanks
Ken