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The Volumetrics section allows you to use an environment volume in your scene, or enable probabilistic sampling for volumetrics.

UI Path: ||Render Settings window|| > Overrides tab > Volumetrics rollout

Use environment volume – When enabled, V-Ray overrides the default Maya environment and renders with the Environment volume loaded below. By clicking on Add New Item you can add one or multiple slots.

Environment volume – The volumetric shader to be used. This is where Volumetric shaders like Aerial Perspective or Environment Fog are connected into the render settings.

Probabilistic volumetrics – When Probabilistic volumetrics is enabled, the volumetrics will select a only few samples (based on the smoke density) along each camera ray, and evaluate the volume lighting at those points. When disabled, the volumetrics will evaluate the lighting at regular intervals using many steps along each camera ray, and thus each pixel will take longer to render, but when it's finished, it will have no noise at all. Enabling Probabilistic volumetrics is particularly useful when using the Progressive Image Sampler in V-Ray, as well as when using complex lighting on the volume. The Probabilistic volumetrics option makes the shading much faster, but also it can result in noisier renders with low sample counts. This mode applies to the Phoenix Simulators, the Volumetric Grid (VRayVolumeGrid) and the Environment Fog. When using V-Ray GPU renderer, this option is not available in the UI, but is turned on and the Num. prob. samples and Num. GI prob. samples are set to 1.

Num. prob. samples – Specifies the number of probabilistic samples to use when Probabilistic volumetrics is enabled. For the best rendering performance, use a higher number of samples for transparent smoke, and lower number for dense smoke. A very high number of samples will converge to standard ray-marching.

Num. GI prob. samples – Specifies the number of probabilistic samples to use for GI rays when Probabilistic volumetrics is enabled. This should be kept as a relatively small number in order to speed up GI calculations.