The boundary article seems to be taking the same approach as is described in the Corona Official

tutorial on clouds, i.e. model the cloud shape and then apply a volume material, as opposed to applying a volume material to a geometric primitive and then generating the fog shape through a shader.

Giona4, this is exactly what I was looking for, thank you so much! I prefer this approach, which is consistent with the latter description I provided above: primitive geometry + control fog shape through shader. Great step-by-step description by the way, thanks again +1