Apple's ringtone container — the same AAC audio as M4A, capped at 40 seconds so iOS recognizes it as a ringtone instead of a song.
M4R is Apple's file format for custom iPhone ringtones, following the same M4-prefix naming convention as M4A (audio) and M4V (video) — informally, MPEG-4 Ringtone. Apple has never published an official expansion of the name, because underneath the extension an M4R file is simply AAC-encoded audio inside an MPEG-4 container, identical to M4A, plus one rule enforced by iOS: a track only appears as a ringtone option in Settings if it carries the .m4r extension and runs 40 seconds or less. That length cap is enforced by the operating system, not a suggestion, so an export longer than 40 seconds fails rather than getting trimmed automatically. The practical effect is that renaming an MP3 or M4A file to .m4r does not make it a ringtone — the audio has to be genuinely re-encoded and trimmed to fit, which is why 'MP3 to ringtone' remains a common search on iPhone despite the underlying audio format barely changing at all.
Once an M4R file is added to your iPhone, it shows up under Settings → Sounds & Haptics → Ringtone, in the Tones section alongside the built-in options. Getting it there takes an extra step beyond just having the file: tapping an M4R in the Files app only previews the audio, it does not install it as a ringtone. The two paths that actually add a track to that Settings list are syncing it through Finder's Tones section on a Mac, or exporting a ringtone directly from GarageBand on iPhone or Mac. Android and Windows do not use M4R at all — Android ringtones are typically MP3 or OGG, so an M4R made on iPhone will not work as a ringtone on another platform without converting it again.
iPhone will not let you set an MP3, WAV, or plain M4A file as a ringtone no matter how it got onto the device — Settings only lists tracks that are actual .m4r files of 40 seconds or less. Converting whatever audio you already have, a song clip, a voice memo, a sound effect, into M4R is the only way it becomes a selectable ringtone at all.
Relevant operations in Filemorph:
None, technically. An M4R file is the same AAC audio inside the same MPEG-4 container as an M4A file. The only differences are the extension and a 40-second length limit, which is how iOS tells a ringtone apart from a regular audio track.
Filemorph converts any MP3 (or other supported audio file) to M4R directly on the iPhone — pick the section you want, trim it to 40 seconds or less, and export, entirely on-device with no upload involved. That export is a finished .m4r file, not yet a ringtone in Settings: Filemorph has no way to write into iOS's Tones library itself, so the last step is syncing the file through Finder on a Mac, or exporting it through GarageBand instead, which is where Apple, not any converter app, actually adds a track to the ringtone list.
iOS only recognizes ringtones in M4R format at 40 seconds or under. A plain MP3, or a full-length M4A, never appears as a ringtone option in Settings no matter how it was added to the phone — converting the clip to M4R is the only way around that restriction.