This is used to render parts of the reflecion probe's surrounding selectively.
If the GameObject's layerMask AND the probe's cullingMask is zero then the game object will be invisible from this probe.
See Layers for more information.
var probe = GetComponent.< ReflectionProbe > (); // Only render objects in the first layer (Default layer) when capturing the probe's texture probe.cullingMask = 1 << 0;
See Also: Camera.cullingMask.