For stories that span multiple movies' eras or are concerned with the events of multiple movies to a roughly equal degree. (Stories primarily tied to a single movie that only casually mention other events should go in another folder.)

For stories which are set in an alternate EqG setting cut off from Equestria (particularly ones in which Twilight Sparkle was always a friend of the others).

And for stories which are generic enough that no era can be determined. This latter definition should be used only as a last resort.