iTunes Match is an iCloud feature that offers access to one’s entire library (if it’s under 25,000 songs) on your Mac or PC, iPhone, iPad and iPod touch. It requires iTunes 10.5.1 and iOS 5.0.1. If you log in to iTunes Match in iTunes on a Mac or PC, your Music library automatically populates with a list of all your music and playlists.
As soon as you double-click a song to play it, the song—whether matched with one of iTunes’ 20 million tracks or uploaded from your original library—will begin streaming. You also have the option of downloading the track by clicking the icon that looks like a cloud with an arrow pointing downward.
On your iOS device (iPad, iPhone or iPod touch), if you tap a song, the Music app begins to stream the device over your Internet connection (Wi-Fi or 3G) while simultaneously downloading the track. With iTunes Match on, you can actually note what has been downloaded locally to the device by the lack of the cloud with downward-pointing-arrow icon next to the track name.
By downloading the song while streaming it, iTunes Match on an iPhone or iPad is attempting to conserve megabytes on an often-limited data plan (the next time you tap the song, nothing needs to be downloaded since it’s already been done). Unfortunately, this can lead to a fuller and fuller iOS device, as any song you tap on gets downloaded to your device. Therefore, it may become necessary to purge the cache of downloaded music on your iOS device.
In order to set your local data back to zero, go to Settings on your iOS device. Scroll down and tap on Music. Then toggle iTunes Match off. With iTunes Match off, returning to the Music app displays all of the local music. Now, if you go back to Settings and tap on General and then Usage, you will see your available and used storage with an audited list of each app and the data it shepherds. If you tap on the Music app in that list, you’ll see what’s managed to be downloaded. First, tap on the Edit button on the top right, then tap the minus sign and tap Delete. This will erase all of the music. At this point, if you revisit your Music app, you’ll see no music! Feel free to return to Settings to re-enable iTunes Match under the Music section of Settings.