Apple Motion 4 - Manual (Page 332)

The order of two objects affected by the Lighten blend mode does not matter.
Screen
Like Lighten, Screen also emphasizes the lightest parts of each overlapping image, except
that the midrange color values of both images are mixed together more evenly.
Blacks in either image allow the overlapping image to show through completely. Darker
midrange values underneath a certain threshold allow more of the overlapping image
to show. Whites from both images show through in the resulting image.
The order of two objects affected by the Screen blend mode does not matter.
332 Chapter 7 Basic Compositing