All simple, check the images.
Top view, corona camera, not orthographic. Render with default corona material as override and volume effect 10 - increased to see result better.
The second image with greatly increased contrast in PS to see where that volume effect produces the strongest result.
Additionally - not sure but seems like in some situations it produces the wrong color, like in a clean scene with sun and default corona material as override - adding volume effect gives some red/purple tint to the scene.
The above is true for at least a few latest DB.
And few questions:
1. aerial perspective still not working with volumetrics, will that be fixed in some nearby future?
2. is it possible to make it true volumetric (checkbox to switch between shader and volumetric)? Or maybe it'll be even if we can plug some map into volumetric material scattering/absorption.