"Persistence is the act of making an Object Reference stick around in the game and not be unloaded. This applies to all references in the game, including things like actor references. Because they continue to stick around, they consume processor time and memory when other references in the same area will have disappeared and unloaded." Off the creation kit site. Unique NPCs, usable bookshelves, display cases, player chests, anything really with a script linked to it.