These files contain data which helps Sound Forge and CD Architect draw the waveform on the screen. They allow files, especially large ones, to be opened quickly. CD Architect will always create SFK files automatically when you put a song into a project. Sound Forge creates SFK files when you open a file in Direct Mode. Sound Forge also creates SFK files when you save a WAV file and answer Yes to "Would you like to reopen the file in Direct Mode?"