Asus Z9PR-D12 - Manual (Page 110)

4-48 Chapter 4: BIOS setup
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.