Apple Final Cut Pro 6 - Manual (Page 1182)

Chapter 19 Compositing and Layering 397
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Normal
Normal is the default composite mode for clips. When a clip uses Normal composite mode,
you can still adjust its transparency by using its Opacity parameter or an alpha channel.
Add
Add emphasizes the whites in each overlapping image, lightening all other overlapping
colors. The color values in every overlapping pixel are added together. The result is that
all overlapping midrange color values are lightened. Blacks from either image are
transparent, while whites in either image are preserved.
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.