I'm guessing that it's a semi-internal system that you can probably find out more about by contacting the OU. If it's referenced in a book, the OU must have some sort of research library. I couldn't find anything on the OU's website, but a quick google search of "OU Document A-94" led to this PDF, which refers to the document which you mention in footnote 31. The footnote reads:
Horav Yisroel Belsky Shlita. Refer to OU Document A-94, see Pischei Halacha (Kashrus) page 118:3 quoting the opinion of the author of the Be’er Moshe (Stern), Bishul Yisroel pages 177-178, Teshuvos from Horav Falk in sefer ibid: pages 29-31.
I don't know for sure the document is available, but that footnote ought to give you some other places to continue your research.
I've emailed the OU and they sent me the document. I've left it in my public Dropbox folder, here. This doesn't answer the question of how their catalog is set up, but this should help you.