Specular Color (RGB)
This is the RGB shading information for Specular "highlights." I most often use this to create Specular "Blooms" and "Stars" to simulate camera filters.

Special (single channel)
This buffer takes its values from LightWave's "Special Buffers" settings (Found under: Surface Editor > Advanced > Special Buffers). I use this to isolate specific areas I may need to work with individually later, such as shadow-catching planes, or surfaces that may suffer from pixel-crawl that might need temporal blurring or other tweaks. (Here, you see it applied to the wood textures of the Walls and Table... textures that I found have a tendency to "buzz/crawl" in certain kinds of camera-moves.)

Luminosity (single channel)
This channel shows how much Luminosity a rendered surface has, taking into account textures, shaders and whatnot. Things with a Luminosity setting of 0 are black, those that are 100% Luminous are white. This is helpful when you've got things like glowing buttons, signs, weapons or the like to isolate them from the rest of the scene to boost/reduce brightness, or add glows just to those items.

Diffuse (single channel)
This channel is a record of the Diffuse amount resulting on rendered surfaces, taking into account textures, shaders and the like.

Specularity (single channel)
This channel shows how Specular a rendered surface is – black for 0% Specularity, white for 100%.

Reflectivity (Single Channel)
This channel shows how Reflective a rendered surface is – black for 0% Reflective, white for 100%.

Transparency (single channel)
This channel shows how Transparent a rendered surface is – black for 0% Transparent, white for 100%.