The beat-unit-tied type indicates a beat-unit within a metronome mark that is tied to the preceding beat-unit. This allows two or more tied notes to be associated with a per-minute value in a metronome mark, whereas the metronome-tied element is restricted to metric relationship marks.
The
<beat-unit-tied>element.The beat-unit-tied type indicates a beat-unit within a metronome mark that is tied to the preceding beat-unit. This allows two or more tied notes to be associated with a per-minute value in a metronome mark, whereas the metronome-tied element is restricted to metric relationship marks.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/beat-unit-tied/