When b is not zero, the color of a texel is combined with the color of a pixel on the screen using the GL_DECAL mode, else if b is zero, the GL_MODULATE mode is used (see the OpenGL documentation for more details). When you want to do lighted textures you normally set the front and back material to white, and use the mode GL_MODULATE, so that this attribute is seldom needed.
b must be a number node.