...

This only applies when Double-Sided is on. When Diffuse Color if off, this sets the back color (the color for the back side). By default, it uses the Diffuse Color but choosing a different color provides you with a way to make the backside of 2D objects appear differently.

...

Transmit Gain

This only applies when Double-Sided is on.  Instead of using the Diffuse Gain for controlling the intensity of the diffuse transmission, we use the Transmit Gain.  If it's 0.0, then the effect is off. Below the Transmit Color is bright blue.

...