NIF file (NetImmerse File) is a format used by all Fallout games using engines derived from the NetImmerse engine first used in The Elder Scrolls III: Morrowind. These include Fallout 3, Fallout: New Vegas (Gamebryo Engine), Fallout 4, and Fallout 76 (Creation Engine).
The format is used to store meshes used by the game, such as characters, creatures, weapons, scenery, landscape, and others. NIF files are typically stored in BSA archives and can be viewed and modified using third-party software such as NifSkope.