An unused voice is one that contains no notes anywhere in the project. Any unused voices are automatically deleted when you close a project, but you cannot manually delete voices once they have been created. You can create as many voices as you want in each staff.
Note
Deleting all notes in a voice does not delete the voice immediately.
If you later want to input notes in a voice that was automatically deleted when you last closed the project, you can create a new voice at any rhythmic position.