Volumetrics

Eevee simulate volumetric scattering by evaluating all volume objects inside the view frustum.

For this it uses several 3D texture which have a high video memory usage. The texture dimensions can be tweaked using the Tile Size and Samples parameters.

Object volumes have some limitations.

Reference

Panel:Properties ‣ Render ‣ Volumetrics
Start
Start distance of the volumetric effect.
End
End distance of the volumetric effect.
Tile Size
Controls the quality of the volumetric effects. Lower size increases video memory usage and quality. This is the size in pixels of a volumetric cell.
Samples
Number of samples to compute volumetric effects. Higher count increases video memory usage and quality. These samples are distributed along the view depth (view Z axis).
Distribution
Blend between linear and exponential sample distribution. Higher values puts more samples near the camera.

Volumetric Lighting

Let the volume scattering scatter light in the scene. Unnecessary if no Volume Scatter is present in the scene.

Light Clamping
Clamp light contribution of the volume scattering effect. Reduces flickering and noise. Set to 0.0 to disable clamping.

Volumetric Shadows

Approximate light absorption of the surrounding volume objects. This makes the volumes more opaque to light. This is a very expensive option and has limitations (link).

Shadow Samples
Number of samples to compute volumetric shadowing.

参考

Limitations.