I have tried a little experiment where I created a cube emitting smoke, then got sphere on another layer to intersect the smokey cube.
I then render it so that the sphere’s layer is set to Mask Layer, but the masking object seems to totaly erase all smoke behind it in a 2D kind of way, rather than simply carve out the volume where it intersects in a 3D kind of way.
Is this because smoke is a kind of 2D post-process? Is there another way you could carve a volume out of a cloud of smoke? I suppose collision might do that in the sense that the smoke would not be able to penetrate the sphere in my case, resulting in a sphere-shaped hole in the smoke cloud…?