References to The Mencius are given in the form "2A6.3". This is because most translations divide the work up into books, really chapters, each with a part A and a part B; or part one and part two. The number six is thus a section and three is a paragraph. Unfortunately, there is absolutely no consistency in this numbering. If you look for 2A6.3 in Legge, it's Book II, part I, chapter VI. Some translators have abandoned the two part numbering, which is presumably how the original work was organised, in favour of conventional Western chapter numbering. Thus 2A is chapter three, which makes looking up references a nightmare. If you use ctext.org, the best internet source for the Chinese texts, each 'book' is denoted by a name followed by I or II.
I have stuck with Van Norden's numbering because it's the easiest; which ever system one uses, checking different versions of the text can be something of a nightmare.