The octave-change element indicates how many octaves to add to get from written pitch to sounding pitch. The octave-change element should be included when using transposition intervals of an octave or more, and should not be present for intervals of less than an octave.
The
<octave-change>element.The octave-change element indicates how many octaves to add to get from written pitch to sounding pitch. The octave-change element should be included when using transposition intervals of an octave or more, and should not be present for intervals of less than an octave.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/octave-change/