From Grub4Dos Wiki

normal usage

The El Torito Bootable CD Specification is about booting from CD, more informations in the wiki.

eltorito.sys is "normally" (for "uncommon" useage see below) a DOS device driver which makes CD-ROM's available to DOS after them have booted. The advantage is that it works for any CD-ROM which has been booted by BIOS even if there is no CD-ROM driver from the producer and no working generic DOS CD-ROM driver. This means it will probable also work for any in the future upcomming new device standards like it's currently working for SATA-, FireWire- and USB-CDROM's as long as bootable by BIOS or BIOS extender. More informations on it's original homepage.

eltorito.sys for CD-ROM's that haven't booted

Originally eltorito.sys was only designed to work for a boot sequence like:

BIOS -> CD-ROM with no-emulation bootloader (for example syslinux) ->
memdisk -> DOS -> eltorito.sys

eltorito.sys continues to use the BIOS to access the device.

I discovered that eltorito.sys can be also used for CD-ROM devices which haven't booted by BIOS or BIOS extender (for example bootable cd loader is a BIOS extender).

One example is at DOS on NTFS.

You can also make the boot sequence like:

BIOS -> bootable harddisk (no matter if IDE, SATA or USB as long as
supported by BIOS or by BIOS extender) -> grub4dos -> DOS

even in this case eltorito.sys can access a CD-ROM.

The relevant example (tested and working) demonstrates this:

title booting FreeDOS from raw image (as harddisk) + FreeDOS installer
CD\nit's up to the operating system to load a device driver for the
virtual CD-ROM, for example:\ndevload eltorito.sys
/D:eltorito\nshsucdx.com /D:eltorito
map --unsafe-boot /FreeDOS.img (hd0)
map /fdfullcd.iso (hd32)
map --hook
chainloader --force (hd0)+1

Another example (not well tested) for a real CD-ROM, you use grub4dos as a BIOS extender (works also if BIOS can not even boot from CD-ROM, doesn't always work).

# - after booting from harddisk
# - grldr in a primary FAT16 or FAT32 partition so this is the grub4dos root device
# - kernel.sys must be in root
title initialize CD-ROM and boot FreeDOS
chainloader /kernel.sys
cd-rom --init
map --hook

here also devload eltorito.sys /D:eltorito ; shsucdx.com /D:eltorito should make the real CD-ROM available to DOS using eltorito.sys.