Suppose you have an iso image that contains a bios bootable partition. You can write this image to a usb disk
In linux you could do this for example with
dd if=pmagic_2013_02_28.iso of/dev/sdc1
Your USB disk is now bootable on computers with bios. However on my UEFI system, the usb disk does not boot. How can you make it EFI bootable?