Well, actually the Distribution Map evaluates for each position if the item is placed or not. In case of grayscale or color maps the Threshold value defines how these values are converted to black/white. Than the Density Falloff effect could be controlled under Areas tab.
Regarding the VRayDistanceTex map "alignment issue":
You're right. To work properly, the Forest object's gizmo should be located in the centre of 3ds Max's World coordinates. We'll see, what we can do in this case. Thanks for your feedback.