https://en.numista.com/catalogue/pieces5630.html
I think it is for those who collect whether the edge lettering is upright or not. If the coin is flat on a table and face (obverse) uppermost, then A is where the edge lettering is also the right way up. So B is when the lettering is upside down (head side still on top).
Just random which is which, as they have the edge made first, then the obverse and reverse pressed, I read on a different topic on here.