Apple Motion 3 - Manual (Page 716)

716 Chapter 8 Working with Particles
In addition, when the 3D checkbox is turned on, the Render Particles, Emission
Latitude, Emission Longitude, and Depth Ordered parameters become available for all
emitter shapes.
Point: This is the simplest emitter shape and is the default shape for newly created
emitters. It specifies a single point of emission for a particle system. There are no
additional parameters for the Point shape.
Line: Particles emerge from a line. Using the onscreen controls (with the Adjust Item
tool) or the Properties tab of the Inspector, you can specify the length and location of
the line. In the Inspector, you can set a specific number of points from which particles
emerge. This emitter shape is good for creating sheets of particles that cascade over a
wide area. The Line shape displays additional parameters.