The Transparency Tool
The Transparency Tool is selected from the Image Toolbox. It behaves differently depending on the currently selected Transparency Tool Options.
The Transparency Tool Options
There are two types of Transparency, which apply different effects.
1. Standard: Simply applies the currently selected brush to screen. It will be transparent to the degree that is selected from within the Tool Options box.
2. Additive: Will add the value of each pixel in the brush to the value of the pixel on the screen. This creates a very different effect. Right clicking will subtract each pixel value.