Slurs, tuplets, and many other features can be concurrent and overlapping within a single musical part. The number-level type distinguishes up to six concurrent objects of the same type. A reading program should be prepared to handle cases where the number-levels stop in an arbitrary order. Different numbers are needed when the features overlap in MusicXML file order. When a number-level value is implied, the value is 1 by default.
<xs:simpleType name="number-level"><xs:annotation><xs:documentation>Slurs, tuplets, and many other features can be concurrent and overlapping within a single musical part. The number-level type distinguishes up to six concurrent objects of the same type. A reading program should be prepared to handle cases where the number-levels stop in an arbitrary order. Different numbers are needed when the features overlap in MusicXML file order. When a number-level value is implied, the value is 1 by default.</xs:documentation></xs:annotation><xs:restriction base="xs:positiveInteger"><xs:minInclusive value="1"/><xs:maxInclusive value="6"/></xs:restriction></xs:simpleType>