Ignore Self Occlusion - when this option is on the object will not occlude itself. It will still be occluded by other objects in the scene.
Ignore other objects - when on, the dirt will affect only the objects themselves, without including contact surfaces and edges. If off, the entire scene geometry is participating for the final result.
Use Transparency - when on, VRayDirt will take into account the opacity of the occluding objects. This can be used, for example, if you want to calculate ambient occlusion from opacity-mapped trees etc. When off (the default), occluding objects are always assumed to be opaque. Note that working with correct opacity is slower, since in that case VRayDirt must examine and evaluate the material on the occluding objects.
Invert normal - this option allows to revert the effect with respect to surface normals - e.g. instead of crevices, open corners will be shaded with the occluded color.
Example: Invert Normal
In the next example, a bitmap is used in the texmap radius slot. Notice that the main radius parameter still has effect - it determines the amount of area where the texmap radius bitmap would blend.
invert normal: off
invert normal: on
Environment occlusion - when this option is checked V-Ray will use the environment when calculating the occlusion of areas that are not occluded by other objects.
Affect Reflection - when checked the ambient reflection will affect the reflection render elements. This can be used to create reflection masks.