Legacy content

Legacy content is the term given to content from an earlier game included among the resources of a later one using the same engine, despite not being used. In many cases, this content is not meant for the setting of the latter game or is a remnant from other, deleted resources.

Gameplay
Legacy content from Fallout found in Fallout 2 game files include audios and images of weapons and armor/clothing, along with NPCs. Content from Fallout 3 remains in the game files of Fallout: New Vegas in a similar way, as does content of Fallout 4 remaining in the files of Fallout 76.