Apple Final Cut Pro 7 - Manual (Page 1202)

The order of two clips affected by the Add composite mode does not matter.
Suggested uses: The Add composite mode is useful for using one image to selectively add
texture to another, based on its lighter areas such as highlights. You can also use Lighten
and Screen to create variations of this effect.
Subtract
Subtract darkens all overlapping colors. Whites in the foreground image go black, while
whites in the background image invert overlapping color values in the foreground image,
creating a negative effect.
Blacks in the foreground image become transparent, while blacks in the background
image are preserved.
Overlapping midrange color values are darkened based on the color of the background
image. In areas where the background is lighter than the foreground, the background
image is darkened. In areas where the background is darker than the foreground, the
colors are inverted.
The order of two clips affected by the Subtract composite mode is important.
Boxes clip on top Monkey clip on top
1202 Chapter 71 Compositing and Layering