The Light Select Render Element represents the lighting contribution from a list of selected lights. It can output their direct contribution (in Raw, Diffuse, Specular or normal Illumination mode) or their Full effect on the scene illumination (including GI, SSS, reflection, refraction, etc). Multiple VRayLightSelect elements can be rendered for a single scene, and lights may be included in more than one VRayLightSelect element. However, a good practice is to have one Light Select element for each light source in your scene. This way, specific lights in the resulting render can be adjusted (color, temperature, brightness, etc.), while compositing without the need of re-render.
For example, by generating a Light Select element for all of the backlights in a scene, an artist may adjust the backlighting of the rendered scene easily in the composite without affecting the rest of the scene's illumination.