Apple Motion 4 - Manual (Page 654)

Using Masks with Particle Systems
As shown in the following images, you can apply masks to the cell source layer of a particle
emitter. The effect of the mask on the cell source is carried through to the emitted particles.
Original source layer Bezier mask applied
to source layer Resulting particle system
You can also apply masks to the emitter object itself.
Rectangle mask (inverted)
applied to the emitter object
For more information on working with masks, see Masking a Layer or Group .
654 Chapter 11 Working with Particles