If the game was purchased digitally via the PlayStation Network, the .RAP file is downloaded silently in the background during the purchase process. It tells the PS3 that the user owns the full game.
Without this key, the RPCS3 emulator cannot decrypt the package files. Attempting to boot a digital version of God of War: Ascension without its corresponding RAP file will result in a black screen or an explicit licensing error. god of war ascension rap file