initramfs module
This module is specific to Debian based distros. Post installation on Debian the initramfs needs to be updated so as to not interrupt the boot process with a error about fsck.ext4 not being found.
Debian specific notes
If you're using live-build to build your ISO and setup the runtime env make sure that you purge the live-* packages on the target system before running this module, since live-config dpkg-diverts update-initramfs and can cause all sorts of fun issues.