I get the impression that the border of the human zone is the heliosphere, which Voyager left in 2012. It makes sense that since the sun is strongly tied to the world just like the moon, that it's boundaries would be important. It would also indicate that if the other planets have their own pruning system and branching timelines that their borders are the heliosphere as well.
Branch timelines are obviously something that other planets have given how the Moon Cell can just casually destroy all timelines for a world. It's a little weird though how CM didn't seem to be aware of them given how he was surprised by the second magic, which is effectively the multiple timelines. It makes me wonder how Zelretch would react if Goetia or FELE Twice's plans had succeeded and there were only one timeline left. Would that effectively destroy the second magic, or would new timelines just start popping up?