Asus Z9PH-D16 - Manual (Page 105)

ASUS Z9PH-D16, Z9PH-D16/FDR 4-47
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.