Shadow linking
You can reduce the rendering time required for a complex scene by linking lights with surfaces so that only a specific light (or group of lights) causes an object (or group of objects) to cast shadows. This technique is called shadow linking.
By default, all objects in the scene are included in the calculation of shadows cast by a light (the light must have either Use Depth Map Shadows or Use Ray Trace Shadows turned on). However, if you use shadow linking to link the light to a group of objects, then only the specified group of objects is included in the shadow calculations.
Shadow linking can help you render scenes more efficiently and quickly. Shadow linking is only applicable when using raytraced shadows and cannot be used in conjunction with depth-mapped shadows.