Smart Playlists in iTunes have always been a powerful way to create specific playlists to meet your needs, from creating a rotating fresh playlist for syncing to an iDevice to creating a specific playlist for a party. Being able to say “give me my music that hasn’t been played in the last month, that is of at least 320kbps and is rated 5 stars” is pretty sweet.
In iTunes 8 and earlier you could create all of these multiple rules and have them applied with a ‘match operator,’ which could be ALL (all rules have to match for a track to be included) or ANY (if any of the single rules apply the track will be included).
iTunes 9 has quietly and substantially expanded the level of complexity that you can create in these rules by allowing you to create nested rules. This lets you build up substantial logic with multiple ANY and ALL match operators being applied.
Read more at The Apple Blog