The Light Select Render Channel represents the lighting contribution from one or more user-selected lights in the scene. Each Light Select channel can output one of the predetermined contribution types, like the overall direct or indirect illumination or the direct or indirect diffuse or specular contribution to the illumination. The Full mode essentially creates a Beauty render for just the included light(s). The Light path expression mode allows for custom contributions to be captured. See the Light Path Expressions page for more information.
Multiple Light Select outputs can be rendered for a single scene, and lights may be included in more than one Light Select render channel.

By using the Light Select render channel, specific lights in the resulting render can be captured and adjusted (color, temperature, brightness, etc.) in a composite without the need to re-render. For example, a Light Select channel for all of the backlights in a scene, an artist may adjust the backlighting in the composite without affecting the rest of the scene's illumination.

UI Path: ||V-Ray Node Editor|| > World > Render Channels > Advanced > V-Ray Light Select

See the Render Channels page to find out where you can find these properties.

Channel Name – Customizable name for the render channel.

Enable Deep Output – Specifies whether to include this render channel in deep images.

Color Mapping – 

Consider For Anti-Aliasing – When enabled, anti-aliasing is used where possible.

Filtering – Applies an image filter to this channel.

Vfb Color Corrections – Applies the post-render color adjustments made from the VFB.

Denoise – Enables the render channel's denoising, provided the Denoiser is enabled.

Mode – Sets what type of lighting contribution is generated in the channel as follows:

Direct illumination – Contains diffuse and specular information from the specified lights.
Direct raw – Lighting within the VRayLightSelect channel behaves as a raw render channel.
Direct diffuse – Lights output only their diffuse contribution and contain no specular information.
Direct specular – Lights output only their specular contribution and contain no diffuse information.
Full – Lights provide full output for all information available in the scene, such as GI, SSS, reflection, refraction, etc.
Indirect – Lights output only the indirect scene lighting.
Indirect diffuse – Lights output only indirect scene lighting that originated from the diffuse contribution of the specified lights.
Indirect specular – Lights output indirect scene lighting that originated from the specular contribution of the specified lights.
Subsurface – Lights output only the subsurface scattering of the light. 
Light path expression – Allows capturing custom light transport paths with regular expressions. 

Light Path Expression – Allows you to enter a Light Path Expression when the Mode is set to Light path expression.

Matte Objects Only – When enabled, it renders only the shadows cast on matte objects illuminated by a pre-selected light.

Lights – Allows you to select one or multiple Light objects/collections and add them to the Light Select render channel. To remove an item from the list, use the Remove button. Use the eyedropper or drag and drop items from the Outliner.