The octave shift type indicates where notes are shifted up or down from their true pitched values because of printing difficulty. Thus a treble clef line noted with 8va will be indicated with an octave-shift down from the pitch data indicated in the notes. A size of 8 indicates one octave; a size of 15 indicates two octaves.
The
<octave-shift>element.The octave shift type indicates where notes are shifted up or down from their true pitched values because of printing difficulty. Thus a treble clef line noted with 8va will be indicated with an octave-shift down from the pitch data indicated in the notes. A size of 8 indicates one octave; a size of 15 indicates two octaves.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/octave-shift/