Apple Motion 4 - Manual (Page 611)

By default, new cells emit one particle per frame in all directions (for 30 fps projects),
and each particle moves 100 pixels per second away from the emitter over a lifetime
of 5 seconds (150 frames in a 30-frames-per-second project).
Particle system at frame 30
Emitter center point
Note: The Initial Number parameter in the Emitter or Particle Cell tab of the Inspector
allows you to change the default behavior so that a particle system begins with a burst
of particles at the first frame. For more information, see
Emitter and Cell Parameters .
The Predictability of Particle Systems
When you create a particle system or modify one of the parameters of an existing particle
system, the path of each particle in that system is immediately calculated and
predetermined. While the number and motion of particles may seem random, they are
actually completely predictable based on that system’s parameters. Playing the same
particle system twice with the same parameters results in exactly the same particle
motion. This means that once you create a particle system that looks right, it is always
the same.
Using Multiple Cells Within a Single Emitter
When you create a particle system from scratch, you don’t have to restrict yourself to
using just one cell. You can create a particle system that emits many different kinds of
overlapping particles by placing multiple cells inside of a single emitter in the Layers tab.
You can add as many cells as you want within a single emitter. Each cell has its own
particle cell parameters that govern how particles from that cell are created. When selected
in the Layers tab, each cell displays its own Particle Cell tab in the Inspector. Particle
systems with multiple cells generate particles from each cell simultaneously, according
to each cell’s parameters.
For an example of using multiple cells within a single emitter, see Example 2: Creating
Animated Pixie Dust .
611 Chapter 11 Working with Particles