Asus Z9PH-D16 FDR User Manual page 105

Boot Override
These items displays the available devices. The device items that appears on the
screen depends on the number of devices installed in the system. Click an item to
start booting from the selected device.
Launch EFI Shell from filesystem device
Attempts to launch EFI Shell application (shellx64.efi) from one of the available
filesystem devices.
ASUS Z9PH-D16 Series 4-47