This can be useful when making explosions that are sitting on the ground rather than floating in mid space. Seed controls the random fractal algorythm, which essentially just gives it a random look, new seed randomly generates a number to go in there. As said before the gizmo only controls the shape, more the transform values of the explosion, such as length, width and height, as well position. Apart from that it is just a helper to combustion. The real settings and controls that really define the look and actions of combustion are in the combustion enviroment menu.
Combustion is located in the enviroment window, go to the render menu and go down to enviroment.
Once you do that a window will pop up, click on the add button to the right and select combustion. This is what combustion looks like, all of the values that control the look and the way it acts are within this menu, so understanding what they all mean and do is very important. To the right is what the combustion menu looks like. Showing all the default settings, now I am going to explain what each and every one of them means in english. By clicking on the button and then clicking on the yellow gizmo it will allow you to choose that gizmo for your combustion.