Bump mapping gives the illusion of highly detailed surface without actually creating more geometry; in other words, it's a shading effect which gives a good approximation to more a detailed surface.
When specifying a texture map, the texture scale (B value) tells Indigo how far the distance is from full black to full white in metres. Since bump mapping is only intended to simulate small surface features, this value will be quite small since it is specified in metres, usually on the order of about 0.002.
See the Texture Maps section for information on texture attributes.
An example material with a grating texture used as a bump map.