help understanding the config.ini settings

[Wolf_Rider]

or at least the -1 levels

Sky=3
Forest=0
VisibilityDistance=3
LandGeom=2
DrawCollisions=1
Water=-1
Effects=1
EffFlags.Light=1
EffFlags.SpriteRender=0
Grass=-1
CordEffect=1
UseFog=0
UseLandCube=1
UseLandConnectedObject=1
LinearObjectManager=1
Roads=1

I understand the 0 = off and 1, 2, 3 etc allow elevation to a max level but the (bolded)-1 level has me scratchings me head.

Could someone offer some clarity?