terrain.center_falloffIsland shape
What it does
Sets the width of the ramp where the centre-land guarantee fades into the noise-made terrain. It's a separate knob from border_falloff this one only touches the middle, not the map edge.
In the real game this is a subtle command the map barely changes across normal values. Only extreme numbers gently break the interior into more islands.
See it on a real map
Reference seed 12345, size 4500. “Try live” opens the generator with the value applied.
default 800
-800 — creates a ring try live →
4000 — much the same map, interior a touch looser try live →
200000 — interior starts fragmenting try live →