MIDI
Definition
.midi (stands for "Musical Instrument Digital Interface") or .mid files, called MIDI files, consist of software instruments readily recognized by the computer and read by the computer in sequences of notes rather than complex audio files, making them smaller than mp3s and other similar file types, though of more computerized quality.
MIDI tracks can be imported into UTAU and appear as a series of notes sung with あ ("ah") by the UTAU voice.