Through the shelf tool “Attract Fluid” I've got my simulation driven from an object. It all works smooth.

But when I try to change the setup I can't find how the “Gas Target Force” actually access the target geometry? I can see that it creates “goal” fields and calculate the force from them but I can't see how any of that lookups the source geo. Where's the magic connection?

If you've done it with the shelf tool it should have created a source apply node in your dop network called “apply_target” (It'll be located after the smoke solver). If you look under the source tab on this node you should see SOP Path and OBJ Path.

But this only creates an object with a “density” and “sdf” field, when are these applied on the fluid?

In my tests, it started affecting the fluid (smoke) from the beginning of the simulation. I managed to change that by animating the intensity of the force, if that's what you want to know…

Cheers