Apple Motion 4 - Manual (Page 339)

While the order of two objects doesn’t affect the overall look of two images blended
using the Hard Mix blend mode, there may be subtle differences.
Difference
Similar to the Subtract blend mode, except that areas of the image that would be severely
darkened by the Subtract blend mode are colorized differently.
The order of two objects affected by the Difference blend mode does not matter.
Exclusion
Similar to the Difference blend mode, except that the resulting image is lighter overall.
Overlapping areas with lighter color values are lightened, while darker overlapping color
values become transparent.
339 Chapter 7 Basic Compositing