Apple Motion 3 - Manual (Page 299)

Chapter 3 Basic Compositing 299
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.