There is a "substrate" line in the tech file that ...
# magic
There is a "substrate" line in the tech file that tells magic to extract the substrate as a net named "$SUB" which lets the name of the substrate be set by a variable in the Tcl interpreter. It's only effect is to keep magic from using its default notation for unnamed nets, which is based on the coordinate position of the lower-leftmost bit of geometry on that net. Since the substrate is just space, its lower left coordinate is at "infinity" as defined by magic, which is a maximum (negative) integer in 32 bits, so it looks confusing as "w_n1073741824_n1073741824#", not to mention just plain ugly and ungainly. So the "$SUB" is just a string substitution. If you specify another value then you should see that value, unless you're doing "extract" instead of "extract all" in which case magic might just use an old extraction file with the VSUBS name in it already.