Smoke and fire effects have to be created somehow and come from somewhere. This is where FumeFX Sources come into play. A source is a helper object that serves as a point of origin. This is where your simulation and all of its essential characteristics stem from.
Each source appears in your scene as an icon, which you can move, re-name, or re-size. Depending on the Source type, the icon can function as the actual source, or be linked to a particle system or object that will act as the source.
Sources operate on a per-voxel basis; so, every voxel that the source touches is defined by the parameters of the source.
Source parameters can also be set to negative values. Negative values will not propagate through the grid like normal smoke or fire; they can only take effect in the volume that the source itself occupies. Negative values in the Fuel Channel will extinguish fuel or fire, whichever is in the affected voxel.
Note: Be aware that the FumeFX simulation treats the source differently than the fluid surrounding it. This may not always be what you want. For example, if you want the source to add smoke into the system, but not to affect the fluid velocity in any way, you must check the Free Flow option so that it simply creates smoke without affecting the other aspects of your simulation.