When Additional Score Options is selected in the Advanced preferences pane, you can display notes in the score as independent notes and edit their independent status. 
When Additional Score Options is selected in the Advanced preferences pane, you can display notes in the score as independent notes and edit their independent status.
Independent notes are not tied to a rhythmic position in the staff, and can be moved freely. This can be used to add grace notes, for example, and for other purposes.
Do one of the following:
Choose Functions > Note Attributes > Independent from the Score Editor menu bar, then choose one of the following settings:
Independent Grace: The note is displayed as a single (independent) grace note.
Independent: The note is displayed independent of the rhythmic context.
Not Independent: The note is displayed in the regular rhythmic context.
Control-click a note head, choose Attributes > Independent from the shortcut menu, then choose an option from the submenu.
Use one of the following key commands:
Not Independent
Independent
Independent Grace