Both of those are variables for the LiveCD environment.
is for the root of the filesystem of the LiveCD that will eventually be compressed. This is where all of the packages get installed during the build process. Use this if you want the files on the LiveCD OS filesystem.
is the root of the CD. Use this if you want the files available without having to boot to the LiveCD or uncompressing the filesystem on the CD. For example, you would copy files to $LIVE_ROOT if you wanted to be able to put the CD in a running system and see them. As in your post above, /EFI, /isolinux, /LiveOS.
There is no variable for the root of the installed (host) system. You retain access to it by using the %post --nochroot
To start simply, you could attempt something like: