Through multitexturing or multipass rendering, Direct3D enables your application to apply detail textures to primitives. Use detail textures to apply scuff marks, bumps, and other surface attributes that give the appearance of realism. You may want to use mipmaps for applying detail textures. See Texture Filtering With Mipmaps.
Detail textures can also be used as depth cues. For instance, suppose your application simulates landing a helicopter. As the helicopter approaches the ground, it is not unusual for the ground textures to be magnified so much that they begin to look fuzzy. In this situation, the user would have difficulty distinguishing the distance to the ground. Adding a detail texture that resembles gravel as the user approaches the ground helps the user derive sufficient depth cues to pilot the helicopter properly.
If the viewer is far from the primitive, you probably don't want the details shown. Be aware, that a primitive may appear brighter when your application does not apply the detail texture. One way to compensate is by applying a light map texture to darken the primitive.