question about the red and grand lines (VERY EARLY on)

OK so i am a netflix convert watching the anime for the first time and i just finished logue town, and one thing i noticed is they showed a map (which maybe they did that in the netflix show too) that clearly showed the grand line on both sides of the red line (because it's perpendicular). is there something special about the part of the grand line that's on the other side of the red line? vs the grand line that splits the east and south blue? They talk about the grand line like this monolith of a place but at least so far, barely even acknowledge that there are parts of it they could just...sail south to reach.

what gives? lol