The philosophy behind the convention is that the ASCII ordering of the file names should agree with the page order from the books and that each file name should refelct as much as possible about the role of the corresponding page.
Each file name (ignoring the extension) is of the form AB_N, where A is the volume number (e.g. 3-1, 4.5), B is a code indicating the section of the volume (introduction, index, ...), and N is the number of the page in its section. I'll be a little more precise about B.
The first letter of B is either "a","c","t", or "x", and these correspond to preliminary pages, the table of contents, the main text, and the index (respectively). For "c" and "t", there is nothing more. For example, page 115 of the main text of SGA 4.5 is "4.5t_115". The index sections are a bit more complex.
In most volumes there are two indexes: notational and terminological. The problem is their order isn't consistent, and so here B is of the form x#C. Here, # is "1" or "2" depending on whether the index is the first or the second, and C is either "t" or "n" depending on whether is terminological or notational. Perhaps it would have been best to lump them both together, but this is the decision I made.
The string B for the preliminary pages is also of the form a#C. Here, C is either "t","p","f", or "i" for title page, preface, forward, or introduction (respectively), and # is again a one-digit number whose sole purpose is to fight the inconsistency of the order that they appear in print.