If the staff-lines element is present, the appearance of each line may be individually specified with a line-detail type. Staff lines are numbered from bottom to top. The print-object attribute allows lines to be hidden within a staff. This is used in special situations such as a widely-spaced percussion staff where a note placed below the higher line is distinct from a note placed above the lower line. Hidden staff lines are included when specifying clef lines and determining display-step / display-octave values, but are not counted as lines for the purposes of the system-layout and staff-layout elements.
The
<line-detail>element.If the staff-lines element is present, the appearance of each line may be individually specified with a line-detail type. Staff lines are numbered from bottom to top. The print-object attribute allows lines to be hidden within a staff. This is used in special situations such as a widely-spaced percussion staff where a note placed below the higher line is distinct from a note placed above the lower line. Hidden staff lines are included when specifying clef lines and determining display-step / display-octave values, but are not counted as lines for the purposes of the system-layout and staff-layout elements.
See
https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/line-detail/