Apple Final Cut Pro 7 - Manual (Page 1203)

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.
Multiply
Multiply emphasizes the darkest parts of each overlapping image, except that midrange
color values from both images are mixed together more evenly. Progressively lighter
regions of overlapping images become increasingly translucent, allowing whichever
image is darker to show through. Whites in either image allow the overlapping image to
show through completely. Blacks from both images are preserved in the resulting image.
The order of clips affected by the Multiply composite mode does not matter.
Suggested uses: The Multiply composite mode is particularly useful in situations where
you want to knock out the white areas of a foreground image and blend the rest of the
image with the colors in the background. For example, if you superimpose a scanned
sheet of handwritten text over a background image using the Multiply composite mode,
the resulting image becomes textured with the darker parts of the foreground.
1203 Chapter 71 Compositing and Layering