Using this option will enable you to combine an image with a background to create the appearance of partial transparency, and/or to create composite images by blending two or more objects with various brush colors. The value of alpha in the color code ranges from 0 to 255, where value 0 represents a fully transparent color, and 255 represents a fully opaque color.
Applying and modifying the transparency
Alpha settings can be used on Shape window in Pen and Brush tools, for blending lines and brushes respectively. Make sure that your Shape window is open. Then find Alpha blending edit boxes on Pen and Brush panels (see the above-left picture). You can change values by dragging the slider up or down, or simply by typing the wanted value in edit boxes.
This option is also enabled in various effects that support color manipulation (see picture beside). In these cases instead of the slider you have a dozen typical values for use.