As a newcomer, it's a good idea to read chapter 2 of the SVRF manual. It covers many fundamental and important rule file concepts.
Environment variables for pathnames are read in an SVRF rule file without additional syntax, but not other things. The Variable statement controls the other things that you can declare using a variable; however, layer names are not among those things. If you want more freedom to declare variables, use the Tcl Verification Format (TVF). Details are in chapter 7 of the SVRF Manual.
Noted, I'll have a good look at those 2 chapters.