Apple Motion 3 - Manual (Page 293)

Chapter 3 Basic Compositing 293
The Gradient object in the bottom group is left unaffected, although the transparency
in the top group caused by the combination of the Add and Stencil Luma blend modes
reveals it in the background.
Other Blend Modes
When you set a group to any of the other available blend modes, each layer nested
within that group is blended according to its blend mode. The resulting image is then
blended with the groups underneath, according to the selected blend mode for that
group. In this example, the topmost group is set to Multiply.
As a result, the combination of the Added Fishes and the Stenciled Swirls objects is
multiplied with the Gradient object in the bottommost group.