Line numbers are counted from the bottom of the staff. They are only needed with the G, F, and C signs in order to position a pitch correctly on the staff. Standard values are 2 for the G sign (treble clef), 4 for the F sign (bass clef), and 3 for the C sign (alto clef). Line values can be used to specify positions outside the staff, such as a C clef positioned in the middle of a grand staff.
The
<line>element.Line numbers are counted from the bottom of the staff. They are only needed with the G, F, and C signs in order to position a pitch correctly on the staff. Standard values are 2 for the G sign (treble clef), 4 for the F sign (bass clef), and 3 for the C sign (alto clef). Line values can be used to specify positions outside the staff, such as a C clef positioned in the middle of a grand staff.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/line/