The Diffusion tool adds a diffuse glow with control over size and emulation of filter grades. Color offset lets you add a tint to the glow effect in order to emulate a tinted diffusion filter.

Size: The range of diffusion, as a percentage of total image size.

Grade: Adjusts the softness of the diffusion grade.

Glow: Amount of luminance/bloom added to highlights.

Highlights Only: The extent to which operation is restricted to the brightest regions of the image.

Highlight Bias: A lower number pushes more of the image into the Highlights region.

Exposure Compensation: Corrects for any resulting changes in overall exposure.

Color: Tints the glow area.