The Free Version, the Primordial Age says to roll 1d8 for each stratum "ignoring the surface
and any stratum with a nexus in it" ...
And that's the only instance of the word "nexus" in the PDF. Google/Internet is no help either... what is a nexus, and how do they get on strata? There's really not much before that page, so I'm curious if there's just a missed stage.