The part-group element indicates groupings of parts in the score, usually indicated by braces and brackets. Braces that are used for multi-staff parts should be defined in the attributes element for that part. The part-group start element appears before the first score-part in the group. The part-group stop element appears after the last score-part in the group.
The number attribute is used to distinguish overlapping and nested part-groups, not the sequence of groups. As with parts, groups can have a name and abbreviation. Values for the child elements are ignored at the stop of a group.
A part-group element is not needed for a single multi-staff part. By default, multi-staff parts include a brace symbol and (if appropriate given the bar-style) common barlines. The symbol formatting for a multi-staff part can be more fully specified using the part-symbol element.
The
<part-group>element.The part-group element indicates groupings of parts in the score, usually indicated by braces and brackets. Braces that are used for multi-staff parts should be defined in the attributes element for that part. The part-group start element appears before the first score-part in the group. The part-group stop element appears after the last score-part in the group.
The number attribute is used to distinguish overlapping and nested part-groups, not the sequence of groups. As with parts, groups can have a name and abbreviation. Values for the child elements are ignored at the stop of a group.
A part-group element is not needed for a single multi-staff part. By default, multi-staff parts include a brace symbol and (if appropriate given the bar-style) common barlines. The symbol formatting for a multi-staff part can be more fully specified using the part-symbol element.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/part-group/