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You can use this tool when there is a staff that is shared by two instruments, and when you want each player to have their part.
When a measure contains two voices, each newly created staff will receive the content of one of the voices.
When a measure contains chords, each newly created staff will receive half of the notes of the chord. If a chord contains an odd number of notes, the middle note will appear in both staves.
You need to place the cursor in the staff you want to split. Then click on the voice button, and click on the button Split Staff.
From here you have access to some options. You can either split the staff into two other staves within the same part. Or you can create those staves into two new parts. You also have the choice to keep the original staff or to remove it.