The M4R file extension is used for iPhone ringtone files. It's essentially an AAC (Advanced Audio Coding) audio file, which is a type of MPEG-4 audio, but renamed with the .m4r extension to be specifically recognized by iTunes and iOS devices as ringtones. These files are typically short audio clips, usually under 40 seconds in length, designed to be used as custom ringtones, text tones, or alert sounds on iPhones. The M4R format allows users to personalize their iPhone's sound notifications beyond the default options provided by Apple. To use an M4R file as a ringtone, it needs to be imported into iTunes (or Finder in newer macOS versions) and then synced to the iPhone. The file format supports metadata, allowing for information like the song title and artist to be displayed when the ringtone is selected. While technically an AAC file, the .m4r extension signals to the operating system and iTunes that it should be treated as a ringtone rather than a general audio file.