The CONFIG option reads the /pmagic/isolinux.cfg and executes it.
It is almost the same as the following option:
The only difference is that the CONFIG option does some kind of reset.
UBCD uses menu.c32 to display the menus, while Parted Magic does use vesamenu.c32 to display the menu.
The following code can't display the Parted Magic menu well:
While the following code works better:
The CONFIG entry resets ISOLI9NUX/SYSLINUX and reads the specified config file just like it would read the isolinux.cfg or the syslinux.cfg file when it starts the first time.
So the CONFIG option is the cleanest way to display the menu of another bootable CD when it has multiple options.
See the next link for the meaning of the other entries:
Download the syslinux package and read the files in /doc to get all information about the options in a config file.