The El Torito standard for bootable CDs provides a 32KB "System Area"
at the beginning of an image.
To create an image that is able to be booted by the BIOS as either a
CD-ROM ("ISO") and as a more HDD-like image (e.g. on a USB flash drive)
it is necessary to have both a standard El Torito boot catalog
containing a HDD-style partition table and boot code.
is intended to be placed in a GPT partition to allow the system to find
in the ISO filesystem later in the image.