When you create a new file it's saved as an OPML file. Of course they could be served by any HTTP server, not just the Radio UserLand server. The OPML format is documented here . Examples of OPML documents Radio UserLand can be used to write any kind of hierarchic document, not just music playlists. Outlines are great for specifications, legal briefs, product plans, presentations, screenplays, diaries, discussion groups, chat systems and stories.
Hierarchies are all over the Internet, outlines are great for creating and browsing hierarchies.
We also think outlines, and outline authoring tools like Radio UserLand, are important for next-generation Internet directories, putting the organizing power in the hands of users is key to making systems like Yahoo and DMOZ really work. When Radio UserLand loads a music file, through commands in the Playlist menu, it adds information about the song to an object database table at user.playlist.songs. Here's a screen shot . As you're listening to music, it updates the information about the song. Here's an example of the XML file.