You can select a bitmap file or to control the Diffuse Level parameter. White pixels in the map leave the diffuse level unchanged. Black pixels reduce the diffuse level to 0. Intermediate values adjust the diffuse level accordingly.
Mapping diffuse level
Top: No mapping
Bottom: Mapping diffuse level with a bitmap
Reducing the Amount of the diffuse level map reduces the map's effect, and increases the effect of the Diffuse Level value on the Basic Parameters rollout. When the Amount is 0 percent, the map isn't used at all.