The base color of the cloud material.
Multiply incoming light by this color as it passes through the cloud.
A multiplier on the actual density value of the volume when calculating diffuse color. Increasing this makes the cloud more opaque.
A multiplier on the actual density value of the volume when calculating shadows. Increasing this makes the shadows darker.
Single scattering phase
How light bounces around inside the cloud. Positive values scatter light forward (along the original light direction). This gives you the "silver lining" effect. Negative values scatter light backwards.
Use internal shadows (self-shadowing).