Apple Final Cut Pro 6 - Manual (Page 1183)

398 Part II Effects
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.
Difference
The Difference composite mode is similar to the Subtract composite mode, except that
areas of the image that would be severely darkened by the Subtract composite mode
are colored differently.
The order of two clips affected by the Difference composite mode does not matter.
Boxes clip on top Monkey clip on top