This determines the order in which the computer has to look in to the devices like CD, floppy or hard disks for a boot sector
The normal order of the search is
Floppy Drive
IDE primary master
IDE primary slave
IDE secondary master
IDE secondary slave
Off-chipset controllers, like SCSI cards, PCI IDE cards or their on-board counterparts. This order may be altered using this option.
Once the boot sector is located, the BIOS will continue with the loading of the operating system files.