The .npk files are the peak files that N-Track uses to draw the wave form on the screen. Whenever N-Track opens a wav file (suppose it's c:\music\vocals.wav), it automatically looks for c:\music\vocals.npk. If the file is there, it uses it to draw the waveform. If it is not there, it generates it for you so next time it won't have to generate it again. So saving off the .npk files will save you a bit of time next time you open that project/song. This could be helpful if you have many tracks as I can see N-Track taking awhile to generate the .npk files for a 20-track song.