Part of the firefly webpage.
Basically, you build a playlist from a query (like artist = “Van Halen” AND artist = “Steve Vai”) and it builds a playlist containing songs that match the criteria. It’s also dynamic.
The wiki entry for the format is here http://wiki.fireflymediaserver.org/Smart_playlists
Adding playlists and manipulating them is here http://wiki.mt-daapd.org/wiki/XML_Client_API#mt-daapd_extensions