Source Controllers give you more control over the values that a source in your simulation will apply to each voxel. The different types of controllers help you to quickly establish a source's linear dependence on various channels in the grid (smoke, fuel, oxygen or temperature) or the distance from an object.
To use this feature, right-click the source controller icon (the square button marked with a minus sign); a menu will appear prompting you to enable/disable or choose a type of controller. You can select Voxel Fuel, Voxel Temperature, Voxel Smoke, Object Distance, Expression or Effector. From here, you can also Copy and Paste parameters from one controller to another.
Right-click the button to select the channel parameters for the selected choice.
Once selected, the button will display a letter corresponding to the choice made:
F = Voxel Fuel
T = Voxel Temperature
S = Voxel Smoke
D = Object Distance
X = Expression
E = Effector
Additionally, once a selection is made, you can then left-click on the button and it will bring up a dialog similar to the one shown below.
For the Voxel Channel choices, you can select Minimum Influence and Maximum Influence values.
Let's say, for example that you assign a Voxel Smoke type source controller to the Fuel channel and then set the Min/Max values to 1/10. This means that, in voxels whose smoke value is less than 1, no fuel will be emitted. And conversely, in voxels whose smoke value is greater than 10, fuel will be fully emitted. For smoke values in between, linear interpolation will be used.
If you use the Expression controller, Max expressions can be evaluated on a per-voxel basis.