Because of the complexities of the algorithms, some BCC filters (for example,
BCC Glow) can mix in color channels from pixels that were originally
transparent. This creates a premultiplied alpha channel which includes
transparency information in the color channels as well as in the alpha
channel. The BCC Premult filter sets the color channels of all transparent
pixels to the selected color, giving the user control of the color that is
mixed in these cases.
When you composite an image with a premultiplied alpha channel over a new
background of a different color, the edges of the alpha channel show a
“fringe” of the old background, because the edges of the alpha channel were
blended with the original background color information. Premultiplied Alpha
Key allows you to key out most of this unwanted color.