FOS file

A FOS file is a save game format used in Fallout 3, Fallout: New Vegas, and Fallout 4.

Gameplay
Each manual save, quicksave, and autosave that the game retains and does not overwrite exists as an individual .fos file. The file sizes of each new game save grows over time when playing the same character.

Location
Like INI files, and unlike record files and other game data, FOS files are found not in any game's install folder but rather within the Documents folder. For each game, this is:
 * C:\Users\[User]\Documents\My Games\Fallout3\Saves\
 * C:\Users\[User]\Documents\My Games\FalloutNV\Saves\
 * C:\Users\[User]\Documents\My Games\Fallout4\Saves\