Apple's official description of the feature is [tragicomically terse][1], iTunes itself gives zero detailed feedback on genius matching, but there are some round about ways to poke at the engines and see if changes to the better can be made.

- iTunes Match will give you a feel for which songs do not instantly match as a side effect of turning the feature on. You initially will have a bulk summary of how many songs matched and which ones were not matched. It's a good bet Apple uses the same type of matching here (perhaps better than genius) - but it's a good start.
- The genius data is not easily readable, so you can't easily [make lists of songs that genius doesn't recognize][2] and also songs it does know, but just can't find enough matches now in _your_ library to make a mix.

I have never seen the design documents for the genius feature, nor someone posting code that dumps the database where apple stores and syncs the genius data to your iTunes and iOS devices, so an authoritative answer might have to wait until code [ends up in a museum][3], but three general aspects of iTunes matching have helped me pick at how things function:

1. Many things go into matching songs - metadata, length, and most likely some fingerprinting of the file as well as an analysis of portions of the sound is likely processed to ensure a match is made of your seed song.
1. These identifying characteristics are uploaded and once the iTunes servers have processed these results, a database is prepared and downloaded to your device (computer iTunes or iOS device) so it can react locally to subsequent match requests.
1. Often a specific song will find matches in one library, but fail to find enough matches when it sits in a different iTunes library. The fact that the error message is the same, doesn't help figure out why a match failed.

I have had some success looking for a song in the iTunes store (when that song is even sold there) and look at the recommended tracks. In practice this lets me judge if the song is known well enough to match in a large library or if iTunes just hasn't pegged that song yet (especially for songs that are _not for sale_ through the Apple storefront as a digital download.)

Since each music library can be wildly different in terms of metadata matching Apple's database and content, it's quite hard to generalize if things are getting better or worse over time. My coverage for genius sure seems to be improving steadily, but that's of little use to someone where genius fails on most songs they care about. It absolutely fails me on new tracks from independent artists and non-mainstream releases that are less than 10 months old.

For now, the process of improving metadata is a trial and error process for me. I hope someone can reverse engineer the database if Apple doesn't decide to add more details or expose more of the matching process, but I'm also not expecting either any time soon.


  [1]: http://support.apple.com/kb/PH1032
  [2]: https://apple.stackexchange.com/questions/14328/can-i-make-an-itunes-playlist-comprised-solely-of-songs-that-lack-itunes-genius
  [3]: http://www.computerhistory.org/