M3U playlists will only support files from 1 memory source at a time. So you can have a playlist comprised of songs residing in the player’s internal memory or the external memory card, but not both. You can have multiple playlists, but sometimes you don’t want to have to re-arrange your music files just to accomodate certain songs in a specific playlist.
PLA format playlists can read (and play) songs from _ both _ memory sources. There are programs/scripts written to convert .m3u to .pla; Google can certainly help you find one that you like.
As the old saying goes, there’s more than 1 way to skin a cat. 
