An additive process adds two values together, such as two colors. When you add colors in 3ds Max, the result is brighter than either of the two original colors.
Additive Opacity
Sphere on the right uses additive opacity.
Additive opacity brightens the colors behind the material by adding the material's colors to the background colors. Additive opacity is good for special effects such as light beams or smoke. You specify the use of additive opacity on the Extended Parameters rollout.
Additive Opacity and the Alpha Channel
By default, additive opacity does not generate an alpha value. In other words, the alpha value is zero, indicating no transparency. This gives correct results with backgrounds in renderings, but if you want to composite objects with additive opacity using video post or a compositing program, you might want to have additive opacity render with transparency. To do so, add the following line to the [Renderer] section of the 3dsmax.ini file, and then restart 3ds Max:
AlphaOutOnAdditive=1
To revert to the default method of rendering additive opacity, in the 3dsmax.ini file, change the value of AlphaOutOnAdditive back to 0 (zero), and then restart 3ds Max.