The degree-alter type represents the chromatic alteration for the current degree. If the degree-type value is alter or subtract, the degree-alter value is relative to the degree already in the chord based on its kind element. If the degree-type value is add, the degree-alter is relative to a dominant chord (major and perfect intervals except for a minor seventh). The plus-minus attribute is used to indicate if plus and minus symbols should be used instead of sharp and flat symbols to display the degree alteration. It is no if not specified.
The
<degree-alter>element.The degree-alter type represents the chromatic alteration for the current degree. If the degree-type value is alter or subtract, the degree-alter value is relative to the degree already in the chord based on its kind element. If the degree-type value is add, the degree-alter is relative to a dominant chord (major and perfect intervals except for a minor seventh). The plus-minus attribute is used to indicate if plus and minus symbols should be used instead of sharp and flat symbols to display the degree alteration. It is no if not specified.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/degree-alter/