The 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 pass and will therefore render as black.
The Reflection Render Element is formed by multiplying the Raw Reflection Render Element by the Reflection Filter Render Element. So while the raw reflection pass gives the full reflection of objects reflecting in the scene, the reflection filter sets how much of that reflection should come through in the composite. The two are multiplied together to create the Reflection pass which gives a true representation of the reflection in the scene.
The Reflection Render Element is a key component in the main Beauty Pass.
