When I started my business I thought that drawing was natural to me, but when it comes to printing? Mmmmm...
Honestly it took me months to understand it, because they don't really explain it to you as it should be, and something I still don't understand. Really if you want to understand it all, you have to work there for years.
Well I try to explain it in my own words.
Let's say there are two standard paper sizes,
"The standard paper size in North America" used primarily in the United States, Canada and in Philippines.
ANSI "America Nation Standards Institute" and then there is
"A-Series International Standard Paper Size". That uses the rest around the world, other words ISO "International Organizations for Standardization"
Well I live in Italy I use ISO "A-Series International Standard Paper Size".