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 clearly not meant for the setting of the latter game, or is an obvious remnant from other, deleted resources.
There are two games in the Fallout series that contain legacy content: Fallout 2 (content from Fallout) and Fallout: New Vegas (content from Fallout 3). In both cases a large amount of content, ranging from items to visual and audio resources, was reused and appears in both games. However, a significant portion was never placed or utilized anywhere in the normal game, but still retains partial resources or even full item/character/etc. entries when the game's data files are examined in detail.
Fallout 2 legacy contentEdit
Most of the legacy content from Fallout resides on audios and images regarding weapons and armor/clothing, along with some NPCs.
Fallout: New Vegas legacy contentEdit
Much like Fallout 2, Fallout: New Vegas uses a large portion of the content from the game immediately preceding it. Much of Fallout 3's location-specific content has been excised, including roughly 95% of the named humanoid NPCs (with some minor exceptions, like Barrows). However, other categories remain largely unaltered, particularly non-humanoid NPCs such as Buckingham, and even including Fallout 3 companions Fawkes and RL-3 (though Dogmeat was specifically removed).
Because Fallout: New Vegas legacy content was simply carried over from Fallout 3, most of its core information remains unchanged, things like weight, value, alignment (for NPCs), and BaseID. Behavior may not remain the same, as certain quests or scripts may no longer be present, or the conditions which would make items or content behave in a certain way no longer achievable. Additionally, while many of these resources remain, often the textures associated with them are gone or changed.