Previous Part Box Symbol

Selects the previous (to the left) symbol in the Part Box. Simple way to switch symbols without having to drag the mouse to the box and click.

Logic Pro part box overview — Apple Support

When Show Advanced Tools is selected in the Advanced preferences pane, you can add notes and other symbols from the Part box in the inspector. The Part box contains a complete collection of score symbols, arranged in groups. You can control which symbols are displayed in the Part box, reorder symbol groups, and open the Part box as a separate, floating window.