From Syslinux Wiki

[edit] About

whichsys.c32 is a COM32 module for Syslinux that determines which command to execute, based on the Syslinux bootloader variant from which it is run.

It is only available in Syslinux 4.02 or later.

[edit] Usage

Usage:    whichsys.c32 [-iso- command] [-pxe- command] [-sys- command]

[edit] Examples

Boot the first hard disk when booted from ISOLINUX, boot the second hard disk (internal hard disk when your USB drive is detected as hard disk 1 by the BIOS) and swap drive numbers when booted from SYSLINUX:

whichsys.c32 -iso- chain.c32 hd0 -sys- chain.c32 hd1 swap

Load a specifc config file for each bootloader:

whichsys.c32 -iso- config iso.cfg -sys- config sys.cfg -pxe- config pxe.cfg