The half-muted element represents the half-muted symbol, which looks like a circle with a plus sign inside. The smufl attribute can be used to distinguish different SMuFL glyphs that have a similar appearance such as brassMuteHalfClosed and guitarHalfOpenPedal. If not present, the default glyph is brassMuteHalfClosed.
The
<half-muted>element.The half-muted element represents the half-muted symbol, which looks like a circle with a plus sign inside. The smufl attribute can be used to distinguish different SMuFL glyphs that have a similar appearance such as brassMuteHalfClosed and guitarHalfOpenPedal. If not present, the default glyph is brassMuteHalfClosed.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/half-muted/