Apple Motion 3 - Manual (Page 719)

Chapter 8 Working with Particles 719
Geometry: Particles emerge from the edge of a shape, defined by a spline object used
as the shape source. The Geometry shape displays additional parameters.
Image: Particles emerge from within an area defined by an image or from only the
edges of the image. The image may or may not have an alpha channel. If it does, the
shape of the alpha channel can also be used to define the emitter shape. The Image
shape displays additional parameters.