The unpitched type represents musical elements that are notated on the staff but lack definite pitch, such as unpitched percussion and speaking voice. If the child elements are not present, the note is placed on the middle line of the staff. This is generally used with a one-line staff. Notes in percussion clef should always use an unpitched element rather than a pitch element.
The
<unpitched>element.The unpitched type represents musical elements that are notated on the staff but lack definite pitch, such as unpitched percussion and speaking voice. If the child elements are not present, the note is placed on the middle line of the staff. This is generally used with a one-line staff. Notes in percussion clef should always use an unpitched element rather than a pitch element.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/unpitched/