Files that contain the .m4b file extension are used to store audio books that are downloaded from the Apple iTunes Store. M4B audio books are copy protected using the Apple "FairPlay" DRM technology.
The Apple FairPlay DRM technology prevents a user from playing the audio books on unauthorized computers and devices. A user may authorize up to five computers on a single iTunes account to play the audio book. If the user wants to play the audio book on more than five computers, he or she must create another account and purchase the book on the second account for use on an additional five computers.
The M4B file format is based on the MPEG-4 container file format. M4B files are usually compressed using AAC encoding. M4B files are similar to M4A files, but the M4B format allows for bookmarking of the files, making the M4B format more suitable for audio books and podcasts.
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