https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/unpitched/
The unpitched element | MusicXML 4.0
elementmusicxml
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/midi-unpitched/
The midi-unpitched element | MusicXML 4.0
the midielementmusicxml