This is why Vray has a tough time with very high changes in brightness because it will never achieve this tolerance. 002 is telling Vray to keep shooting more samples and sub divide pixels until adjacent pixels are within this range. This is the maximum allowed difference in pixel value on a floating point scale. Lower values will increase the quality by not allowing Vray to lower the subdivisions for materials and lights deemed low importance, (distant lights and objects, darker objects)
