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.