FOS file

A FOS file (Fallout Save) is a save game format used in Fallout 3, Fallout: New Vegas, and Fallout 4. 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, so if a new save slot is used every time the size of the Saves folder can grow very large. This can contribute to lag and unresponsiveness in certain menus, which can be alleviated by manually moving older saves outside of the folder.

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\