Sometimes a player doesn't want to make a single character, but instead an entire faction. These can be corporations, secret guilds or marauding armies. There can be times when, while fighting a horde of the EVIL EMPIRE's faceless mooks when a player will wonder, "wait a second, what do these guys look like? Do they use swords or guns? And that one unique one with a special name. You mentioned it before, but I forgot what was special about it?" So here's the place for you to describe all your mooks, once and for all.
There is no guideline for creating a faction, since it's pretty much up to the creating player.
ACRONYM organizations, due to their nature, do not go here. They are made up of many players, and have their own thread for this sort of thing.
Sometimes, people play characters of a certain race enough times, or have so many characters of a single race that it becomes more convenient for them to simply post their racial abilities in one post and refer to it continually, especially if that race is completely made up, or different to that in the public consciousness. These people (these sad, sad people) sometimes make absolutely MASSIVE posts, detailing the minutae of their made-up race's made-up biology and genetics. Their sad, nerdy exercises in futility go here, along with their name, so that people can ridicule them, and also so that people can distinguish between say, John Doe's Vampires, and Hong Gil-Dong's Vampires, which are almost exactly the same, except Gil-Dong's Vampires sparkle in the sunlight. Races in this index are usually open for other people to make characters from (as if people would use OTHER people's ideas instead of their own!) unless otherwise noted in the post.