You create a combustion apparatus -- or "gizmo" -- to position the effect in your scene and to define the maximum boundaries of the effect. The apparatus is a Helper object found in the Atmospheric Apparatus sub-category.
There are three kinds of apparati: BoxGizmo, SphereGizmo, and CylGizmo.
You can move, rotate, and scale the apparatus, but you cannot apply modifiers.
Using non-uniform scale is a good way to change the shape of the apparatus for effects other than spheres and hemispheres. (You will see a warning when you use this transform. Because you don't modify atmospheric apparati, you can safely ignore the warning.)
The final two parameters for all atmospheric apparati set a random seed to create variation when you assign multiple combustion apparatus objects to the same effect. If more than one apparatus uses the same seed and same atmospheric effect, they will produce nearly identical results.
Seed: Sets a base value used to generate the Combustion effect. Each apparatus in the scene should have a different seed.
New Seed: Click to generate a random number automatically and place it in the seed field.
This gizmo has the shape of a box.
Length, Width, and Height: Set the dimensions of the box gizmo.
To create the BoxGizmo, drag in a viewport to define the initial length and width, then release the mouse and drag to set the initial height. Click to end BoxGizmo creation.
The SphereGizmo is shaped like a sphere or hemisphere.
Radius: Sets the radius of the default sphere. Drag in any viewport to define the initial radius, then adjust it with the spinner.
Hemisphere: When set, the bottom half of the SphereGizmo is discarded, creating a hemisphere.
The CylGizmo is shaped like a cylinder.
Radius and Height: Set the dimensions of the cylinder gizmo.
To create the CylGizmo, drag in a viewport to define the initial radius, then release the mouse and drag to set the initial height. Click to end CylGizmo creation.