In the forums for the different known seasons, stories will be posted that take place with in a certain/specific season. Alternate Universe (AU) fics may also be posted in the seasonal forums.

Alternate Universe is defined as: A story that sharply deviates from the canon of the show. Most fanfics deviate in some way from the canon of a universe, but AU deviates in major ways. For example Joyce never dies in season 5, Spike never gets chipped, Angel never returns from Hell...Buffy getting pregnant can also fall under this definition.

So a story that has Glory but no Dawn existed and Joyce doesn't die, then this fic would be set in season 5 and deemed AU.