Random-Number Seed

flag  Random:setSeed   (default = off)
Indicates whether a user-set seed should be used every time the Pythia::init routine is called. If off, the random number generator is initialized with its default seed at the beginning of the run, and never again. If on, each new Pythia::init call (should several be made in the same run) results in the random number being re-initialized, thereby possibly starting over with the same sequence, if you do not watch out.

mode  Random:seed   (default = -1; maximum = 900000000)
The seed to be used, if setSeed is on.
A negative value gives the default seed,
a value 0 gives a random seed based on the time, and
a value between 1 and 900,000,000 a unique different random number sequence.

For more on random numbers see here. This includes methods to save and restore the state of the generator, and some preprogrammed methods to generate non-uniform random numbers.