Diffusion creates atmosphere by reducing contrast while creating a glow around highlights or shadows using an extensive texture library.

Diffusion.

To select a preset, choose one from the Presets tab.

Determines the blend mode to be used to create the diffusion effect.

The diffusion is added to your image.

The diffusion is combined with the image using a Screen blend mode. This looks kind of like the Add blend mode, but highlights are retained.

Sets the intensity of the diffusion.

The Color parameter sets the color of the diffusion through the use of a standard color picker. The default color is white.

Go to the Color Correct node to see how the Color Correct controls work.

Selects the texture which will be used to add diffusion to the image.

Textures can be used as the source of the diffusion as well as combined with a matte using a variety of Blend modes. Go to Blend Modes for explanations of the various modes.

I like the Multiply blend mode for combining textures with the matte because it only puts the texture within the areas of the generated matte.

The texture can be manipulated using a Transform. Go to the Transform node to see how it works.

A matte can be used to create the diffusion effect. Go to the Matte section of Common Node Controls to see how the Matte controls work.