What Additive Transparency looks like when used

Additive Transparency – 256 colours - The darker colours in the texture are the most transparent, black being fully transparent. Very good for cobwebs.

Additive Transparency works by making the darker content of the image more transparent.
It works similar to the screen layer-blending mode in Photoshop.
The texture basically affects what is seen behind it. The result is always a lighter colour. Black however, leaves the colour unchanged with white produces white.
The effect is similar to projecting photographic slides.

These two textures above, when set to Additive give an effect like this.

Notice how the strong hues keep there opacity throughout but the black blend has disappeared.
Keep in mind that textures can offset slightly as you can see on the image above. There is a thin line of white to the left. The best way to solve this is make sure you have a transparent boarder on the texture or crop that part off the face of the brush.

Here's an example with a photograph, both in colour and greyscale.

Conclusion

That's about it really, good for things like cobwebs. You can also do animation with it.

DaddyCool