The Reflection Filter Render Element stores reflection information calculated from the materialsreflection values in the scene. Surfaces with no reflection values set in its material(s) will contain no information in the render pass and will therefore render as black.
While the V-Ray Raw Reflection Render Element gives the full reflection of objects reflecting in the scene, V-Ray Reflection Filter shows 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 and produce V-Ray Reflection. Using these component parts of the reflection, you can better fine tune the reflection in your final composite.
Reflection element is a key component in the main Beauty Pass.

The Reflection Render Element is useful for changing the appearance of Reflective elements after rending in a compositing or image editing software. Below are a couple of examples of their use.

The Reflection Render Element

Original Beauty Composite

Brightened Reflections

Warmed Reflections

Compositing Formula

The Reflect render element is a product of  V-Ray Raw Reflection * V-Ray Reflection Filter.