Ideally, you should work in Volumetric mode all the time and switch to Volumetric Geometry only when generating the Render Element passes.
The Volumetric Geometry mode tends to be slower to render. The Volumetric Geometry mode does not work with Probabilistic Volumetrics. you use an Aerial Perspective and a Volume Grid Simulation - if the mode is set to Volumetric, one would be entirely behind the other in the rendered image. Blending of multiple atmospheric effects in the scene - e.g. 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 Render Elements page. The Volumetric Geometry mode is provided as an option for the following reasons: 
Volumetric vs Volumetric Geometry mode when rendering on the CPU
This page provides some useful tips for rendering volumetric effects.
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