For unpitched instruments, the midi-unpitched element specifies a MIDI 1.0 note number ranging from 1 to 128. It is usually used with MIDI banks for percussion. Note that MIDI 1.0 note numbers are generally specified from 0 to 127 rather than the 1 to 128 numbering used in this element.
The
<midi-unpitched>element.For unpitched instruments, the midi-unpitched element specifies a MIDI 1.0 note number ranging from 1 to 128. It is usually used with MIDI banks for percussion. Note that MIDI 1.0 note numbers are generally specified from 0 to 127 rather than the 1 to 128 numbering used in this element.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/midi-unpitched/