Surface Settings

Surface Settings allow for manipulation of objects based on the surface being generated on

Align To Surface - Align objects vertical direction to the normal of the surface below

Limit By Steepness - Range to limit generation on surfaces with slopes outside of the range

Vertical Sink Depth - The distance to sink objects vertically. Relative to the height of the object

Slide Down Slopes - Slide objects down surfaces

Align To Surface Normal

Alignment strength determines how much the objectaligns to the direction of the surface normal
Trees aligning to the normal of the surface below them

Limit By Steepness

Limiting by steepness will only generate objects on surfaces that have a slope within the range specified
A tree created on surface with an angle greater than 2.6

Vertical Sinking

Vertical Sinking will sink your objects into the surface it is being created on
A tree being 'sunk' into position

Slide Down Slopes

Slide Down Slopes will move objects along the surface going downward 
Rocks being slid down slopes