Apple Motion 4 - Manual (Page 641)

Birth Rate: A slider that defines the birth rate of the cell. In other words, this parameter
defines how many particles of this cell emerge from the emitter every second. Higher
values create denser particle effects.
Low birth rate particle system High birth rate particle system
Birth Rate Randomness: A slider that defines an amount of variance in the Birth Rate of
generated particles. A value of 0 results in no variance—particles emerge from the emitter
at the same rate. A value greater than 0 introduces a variance defined by the Birth Rate
parameter, plus or minus a random value falling within the Birth Rate Randomness.
Initial Number: A slider that defines the initial number of particles. This parameter
determines how many particles of this cell appear at the first frame of a particle effect.
The result is an initial burst of particles that eventually evens out according to the Birth
Rate parameter.
Initial Number set to 5 (frame 3) Initial Number set to 20 (frame 3)
641 Chapter 11 Working with Particles