There is only one file type that uses the .M4P extension:
iTunes Music Store Audio File
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.
Updated: July 20, 2015