I don't know much about playlist formats but PLA seem to be the Windows Phone default. When the phone is connected to a computer, If you right-click on a music folder with tracks in it, you can select "Create Playlist" and a .pla file is what the phone creates (or what the computer creates on the phone). Ironically the phone creates the .pla file in the Music folder instead of placing it in the Playlist folder. ??? My music app, Monochrome Audioplayer, picks the playlist up in both locations so I think I will just sync everything to the Music folder from now on and forget about the Playlist folder.
http://webcazine.com/4443/how-to-create ... indows-pc/
A Wiki I found shows various playlist formats including the PLA. I am not advocating the PLA. Actually it looks like M3U is the way to go since my brief study indicates it is the most popular playlist format used.
http://en.wikipedia.org/wiki/Playlist