You can output the merged foreground and background as a final composite, or you can output the premultiplied or unpremultiplied foreground for compositing elsewhere in your node tree. In the output mode dropdown menu, select:
• composite - to output a merged layer of the background input with the extracted subject.
• premultiplied - to output the extracted subject premultiplied by the final matte.
• unpremultiplied - to output the extracted subject and final matte unpremultiplied.