Complex sentence: The term is applied also to sections of large vocal works (such as opera, oratorio, or musical) when the written or printed score for such a work designates the titles of the pieces with sequential numbers, hence the aptness of the term.
Split into simpler sentences: The term is related to extended vocal works (such as opera, oratorio, or musical). The written or printed score for such a work often designates the titles of the pieces with sequential numbers, hence the aptness of the term.