This tab controls the way VRay interprets units. Since lengths are measured in Softimage units VRay needs to know how they are related to metric scale. Parameters in this tab affect directly VRay physical camera and lights.
Meters scale -The number by which a 3d distance in Softimage units must be multiplied to convert it into meters.
Photometric scale - The number by which the power of photometric lights should be scaled when rendering. Smaller values will increase VRay physical camera's image brightness if exposure is turned on.