Edit Brush Dialog Box - Transparency Tab
Flat transparency also changes the transparency of the line. (Other transparency types have no effect on the line.) You can also use semitransparent objects in creating the brush. These sliders let you override any transparency applied to the brush.
Transparency
0% - no change
positive values - increase the transparency. Has no effect on any fully transparent objects.
negative values - decrease the transparency. Affects only to transparent or semitransparent objects.
Pressure
This has an effect only when using a pressure-sensitive input device such as a graphics tablet. The transparency is controlled by the pressure you applied when creating the line. the slider sets the range within which transparency can vary.