The songs in your iTunes Library may all seem to be essentially the same. They're audio files; why would they be different? But, if you look closely, you'll find out that while many of the songs are the same kind of audio file, others differ in some pretty major ways.
Finding a Song's Filetype
If you have you the "Kind" column turned on in your iTunes library (View menu -> View Options -> Kind), or view a song's file information (command-I on a Mac, Control-I on a PC), you may notice that some songs have many different kinds of information attached to them. In the Kind field, some are MPEG audio files, others are purchased, and yet another group is protected. The question is: what do these differences means? Why are some files "purchased" and others "protected"?
The type of file a song is has to do with how it got into your iTunes library. Songs that you rip from CD will show up in iTunes based on your import settings, while songs you buy from the iTunes Store or Amazon or elsewhere may be something else entirely. Here are some of the most common kinds of files you'll find in your iTunes library and what each one means:
Matched AAC audio file: This is a standard AAC audio file, except that you downloaded it to your computer or iOS device from your iCloud account, using iTunes Match.
Protected AAC audio file: A protected AAC file was the default filetype for songs purchased from the iTunes Store prior to the introduction of the DRM-free iTunes Plus format in April 2009. It is noted as being protected because the DRM built into the file prevents it from being copied/shared beyond the iTunes account it's associated with.
Purchased AAC audio file: A purchased AAC file is what a protected AAC file becomes when it's been upgraded to the iTunes Plus format. This file was still purchased at the iTunes Store, but it no longer has the DRM-based copy restrictions. All songs at the iTunes Store sold after April 2009 are in the Purchased AAC audio file format.
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