What does the '--' do in a normal syslinux config ? I used to see "quiet" after that , but don't know why.
Here's an example from Ubuntu 12.04:
label install
menu label ^Install
menu default
kernel ubuntu-installer/amd64/linux
append vga=788 initrd=ubuntu-installer/amd64/initrd.gz -- quiet
Anything that is passed after the '--' are appended to the default options for the installed system. Everything before the '--' is only used by the installer itself.
Source: http://www.syslinux.org/archives/2009-December/013978.html
When the installation is finished you can find the options in the /etc/default/grub file at the line starting with GRUB_CMDLINE_LINUX_DEFAULT.
If you edit this line you wille have to type update-grub so the content of this line is installed in the boot loader (used when launching the Linux kernel).
Many commandline tools take arguments. There are one-letter, short, arguments and ... long arguments. They change the default behaviour of such a tool.
Short arguments are prefixed with a single dash -
Long arguments are prefixed with a double dash --
Short arguments can be combined into ls -l -t -r is the same as ls -ltr.
Long arguments need to be distinguished from combined singles, and to do that, a long argument is prefixed with a double dash ls -l -t --reverse or ls -lt --reverse
Long arguments are easier to remember, short ones faster to type.
Most commands have a manual page which explains these arguments in detail. Eg. man ls for the manual page of the ls command which I used in my examples above.
-- in a configuration file, not the command line. GNU getopt(1) and getopt(3) use -- to signal the end of options. Everything after it is interpreted as an argument. E.g. grep -- -E file looks for the string -E in file (even though -E is a grep option). This could be what's happening here if syslinux (ab)uses getopt(3) to parse its configuration files. (or if it simulates the behaviour)
-- but I can't answer the question in spirit because I don't know what the heck it's doing in a SysLinux append line. :)