MIDI
A decades-old standard that stores which notes were played rather than the sound itself, so anything can play them back.
MIDI is the difference between a recipe and a finished meal. An audio file stores the actual sound of a piano. A MIDI file stores only the instructions: this note, at this moment, held this long, hit this hard. No sound at all. Hand those instructions to a piano sound and you get a piano, hand them to a synthesiser and you get a synthesiser, and you can change any single note afterwards without re-recording anything.
The standard dates back to 1983 and is still everywhere, which is why “MIDI import” shows up in release notes as an unglamorous but important feature. It is the format that lets you bring a part you played yourself into a tool, or take what a tool generated out and edit it somewhere else. Without it, AI music output is a finished mix you can only accept or discard.