Slope would usually be calculated from the planet surface normal, so you can have slope-dependent surfacing anywhere on the planet, and the slope calculation will still be correct, relative to a flat planet. "Use Y For Slope" makes it calculate from the Y axis, meaning it will change relative to the planet surface as you get further from the coordinate origin. You can use this for interesting planet-wide effects that are not relative to the surface direction/normal, such as ice caps, or equatorial bands.
- Oshyan
