@hunse can say more, but the idea was to have two seeds: one for building the model, but also one that seeds the simulator's RNG. The idea being that you might want to generate the same model, but then still have variability when you run it to make sure that that it's robust to injected noise, or some other source of noise in the simulation. We were thinking about making a nonlinearity that generates random values, and the simulator seed would be used for instances of that nonlinearity.