The Volumetric Geometry mode is provided as an option for the following reasons:
Generation of Render Elements for compositing - for a complete list of the supported Render Elements in both Volumetric and Volumetric Geometry mode, please check the V-Ray Render Elements Support page.
Blending of multiple atmospheric effects in the scene - e.g. you use an Aerial Perspective and a Phoenix FD Simulation - if the mode is set to Volumetric, one would be entirely behind the other in the rendered image.
The Volumetric Geometry mode tends to be slower to render. Ideally, you should work in Volumetric mode all the time and switch to Volumetric Geometry only when generating the Render Element passes.
The Approximate and Approximate+Shadows Scattering options under Volumetric Render Settings → Smoke Color are not supported in Volumetric Geometry mode. It would default to Disabled scattering.
You may need to increase the V-Ray Settings → Global Switches → Advanced toggle → Max.Transparency Levels option which will slow-down rendering. When rendering in Volumetric Geometry mode, procedural geometry made up from multiple transparent layers is generated at render-time. Think of it as sprites used in video games - a sequence of slices facing the camera.
The default 3ds Max Scanline renderer is not supported.
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When rendering with V-Ray GPU, the Render Mode does not matter - Render Elements are generated and the rendering speed is the same in both the Volumetric and Volumetric Geometry modes.
