These are file allocation tables. They act as an index or map, telling the game engine exactly where to look inside the corresponding .dat file to pull a specific sound.
Specifically, the and SoundEnglish.fat files are critical components that house the English voice acting and audio data. If these files are missing, corrupted, or incompatible, you might find yourself in a silent Rook Island, unable to hear dialogue, gunfire, or ambient sounds. These are file allocation tables