This is a list of NPCs that are "extra" (in the sense of movies/theater). They serve no special purpose except as "ambience". In the game, it just means that they do not fall under the other NPC types; meaning they do not provide any trading, crafting, no quest involvement, or any other function.
This category shows the general NPCs that are not specific to a particular campaign, expansion or special event - in other words, those that can be found in the Underworld or the Fissure of Woe. The Battle Isles NPCs are listed in Category:General NPCs (Core).