Protected .AAC audio file downloaded from the iTunes Music Store; incorporates Apple Computer's "Fairplay" DRM copy protection, which limits the number of computers the file can be played on.In order to play a protected M4P file, you must authorize your computer using Apple iTunes; this is done by entering a username and password for the account that downloaded the file; up to five computers can be authorized with the same account.A proprietary version of AAC in MP4 with Digital Rights Management developed by Apple for use in music downloaded from their iTunes Music Store. Protected .AAC audio file downloaded from the iTunes Music Store incorporates Apple Computer's "Fairplay" DRM copy protection, which limits the number of computers the file can be played on. In order to play a protected M4P file, you must authorize your computer using Apple iTunes; this is done by entering a username and password for the account that downloaded the file; up to five computers can be authorized with the same account.