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 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.