Multiple part-link elements can link a condensed part within a score file to multiple MusicXML parts files. For example, a "Clarinet 1 and 2" part in a score file could link to separate "Clarinet 1" and "Clarinet 2" part files. The instrument-link type distinguish which of the score-instruments within a score-part are in which part file. The instrument-link id attribute refers to a score-instrument id attribute.
The
<instrument-link>element.Multiple part-link elements can link a condensed part within a score file to multiple MusicXML parts files. For example, a "Clarinet 1 and 2" part in a score file could link to separate "Clarinet 1" and "Clarinet 2" part files. The instrument-link type distinguish which of the score-instruments within a score-part are in which part file. The instrument-link id attribute refers to a score-instrument id attribute.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/instrument-link/