The optional wiring is if you want to use a footswitch for your speed and kill switches with LEDs to indicate whether they’re engaged or not ie on or off rather than a toggle switch so it’s just showing you how to connect speed and kill footswitches as an option
The ground connection to those switches simply switches the LEDs on or off by connecting the LED cathode negative pin to ground to visually indicate whether they’re engaged ie on or off
As Barry says you can use any ground point you don’t have to use the pad shown in the diagram you can connect them to the DC jack ground lug or the in and out jack sleeve lugs whatever is easiest for you as long as all ground points are connected to ground
Are you going to use footswitches for the speed and kill switches?
It’s down to personal preference so if you think you’ll use them go for it we’re here to help so if you are unsure just ask