The Raw Reflection Render Element stores reflection information calculated from the materials’ reflection values in the scene. Surfaces with no reflection values set in their materials will contain no information in the render element, which means these areas will render as black. When the Raw Reflection Render Element is multiplied by the Reflection Filter, the Reflection Render Element is produced.
V-Ray Raw Reflection gives the full reflection of objects reflecting in the scene, while the reflection filter V-Ray Reflection Filter sets how much of that reflection should come through in the composite. In other words, the filter defines the strength of the reflection, while the raw image defines what is being reflected in the image. When these two elements are multiplied, the true level of reflection is given as the V-Ray Reflection Render Element. By using these component parts of the reflection, you can fine-tune the reflection in your final composite.