The distance that a ray travels before Unity considers it to be unoccluded when calculating ambient occlusion in lightmaps. (Editor only).
Ambient occlusion (AO) is an effect that approximates how much ambient light a given point on a surface receives. When you apply AO to a Scene, areas of concavity - such as creases, holes and adjacent surfaces - appear darker.
Unity calculates AO by casting rays in a hemisphere from lightmapped texels. This value determines the length that a ray travels before Unity considers it to be unoccluded.
Larger values produce longer rays, which increases the search radius. This means that wider areas of concavity will appear darker. Lower values produce shorter rays, which means that areas of concavity must be tighter in order to appear darker. A value of 0 produces an infinitely long ray, meaning all geometry is considered. The default value is 1.
This setting affects lightmaps generated by the Baked Global Illumination system. Note that this property relates only to baked AO, and has no effect on any real-time AO post-processing effects.
When Unity serializes this LightingSettings object as a Lighting Settings Asset, this property corresponds to the Max Distance property in the Lighting Settings Asset Inspector.
Additional resources: Lighting Settings Asset.