This parameter determines what to do with any embedded alpha in the original source image. You need this if you are doing multiple keys on different parts of the image with the View output set to Intermediate Result.
• Ignore - this does not add any embedded alpha to the screen matte.
• Add To Inside Mask - the embedded alpha is added to the inside mask. You should select this when multipass keying with Output View set to Intermediate Result.
• Normal - the embedded alpha is used to composite the image.