Some text of the syslinux documentation:
LOCALBOOT type [ISOLINUX, PXELINUX]
On PXELINUX, specifying "LOCALBOOT 0" instead of a "KERNEL"
option means invoking this particular label will cause a local
disk boot instead of booting a kernel.
The argument 0 means perform a normal boot. The argument 4
will perform a local boot with the Universal Network Driver
Interface (UNDI) driver still resident in memory. Finally,
the argument 5 will perform a local boot with the entire PXE
stack, including the UNDI driver, still resident in memory.
All other values are undefined. If you don't know what the
UNDI or PXE stacks are, don't worry -- you don't want them,
just specify 0.
On ISOLINUX, the "type" specifies the local drive number to
boot from; 0x00 is the primary floppy drive and 0x80 is the
primary hard drive. The special value -1 causes ISOLINUX to
report failure to the BIOS, which, on recent BIOSes, should
mean that the next boot device in the boot sequence should be
MENU LABEL *Boot Next Device
This only works on newer BIOSes.
Download also the last syslinux version. It contains a lot of extra parameters that you can pass to the chain.c32 command (to boot other hard disks).