What are B-15 XLT lenses on rayban sunglasses?

From a RayBan customer service employee:
The B15 is a brown lens and the G15 is a grey/green lens. Both lenses allow 15% light through the lens. It also blocks the UV reds and UVB rays.

From the bottom of a website( http://www.bardoptical.com/html/sunglasses.html ):
G-15 Lenses Historical Perspective In 1951, in response to a Navy Air Corps requirement, Bausch & Lomb began developing a truly neutral sunglass lens, called the N-15. This lens was reformulated slightly to make it more suitable for prescription use and renamed the G-15. The military services were so impressed that they wrote a complete set of specifications around the characteristics of the G-15 lenses. How the Lens Works Human eyes do not respond equally to all colors in the visual spectrum. The eyes are more sensitive to green and yellow wavelengths and are less sensitive to red and blue. Based on this knowledge, the G-15 lens was formulated to emphasize the colors that our eyes see most easily and de-emphasize less useful colors. The G-15 lens absorbs 85% of visibile light, transmitting only 15%. The light transmission curve of the G-15 (showing the amount of light that is allowed to pass through the lens) is similar to the color sensitivity of the human eye. What this means is that the lens transmits colors in the same way tha tour eyes are sensitive to colors. This results in natural vision - true colors that are easy on the eye. The G-15 lens is made of impact resistant optical quality glass. It provides 100% Ultraviolet (UV) protection, is naturally scratch resistant, and has outstanding stability with virtually no time-related fading or alteration of the tint.