Adobe After Effects CS3 - Manual (Page 359)

AFTER EFFECTS CS3
User Guide
354
About randomness and random seeds
Because true randomness is not repeatable, many effects simulate randomness by using a calculation that generates
seemingly random results for each value of a Random Seed property. Multiple instances of the same effect will give
the same results if all of their settings—including the Random Seed property values—are the same. This allows you
to get predictable, deterministic results, while still achieving the appearance of randomness.
Changing the Random Seed value doesn’t make things more or less random; it only makes them seem random in a
different way.
You can add randomness to any property with the expressions in the Random Numbers category.
See also
“Random Numbers methods” on page 563