Asus Z9NA-D6 - Manual (Page 107)

ASUS Z9NA-D6 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
This item is for launching the EFI Shell application from one of the available
filesystem devices.