I know that one. I was wondering more about the source for all those other numbers and who's responsible for coming up with them.
I know some of the numbers come from that Alan Moore Captain Britain series, others from the Exiles comics and various other reality hopping hero stories like Howard the Duck's appearance in Marvel Zombies. Most though, come from various "Official Handbook to the Marvel Universe" issues. This
wikipedia page list the sources for most of the number designations. I don't know that there is an official system for it (the people in charge, Joe Quesada and Tom Brevoort are not actually fans of the system, feeling it's far too derivative of DC and an era of DC they didn't like), but most of the numbers seem to have a basis in the date they were created.