XXXX releases can be a few things. - Non-scene releases: basically just releases that are made by people who are not in the scene (release groups) - Nukes: a release that is nuked for any reasons will be made XXXX A numbered release is a release that has no problems and is properly released by the scene. Our numbering is screwed up by the way, so don't go searching for releases via numbering.
traditionally xxxx indicates a rom of something not commercially available (like the pokemon distribution roms).
Nope. ROM sites and release sites like GBAtemp and DS-Scene make up the numbering. Everyone is different at the moment.