Epidimiology module SEIR model why the observation follow distribution binomial_dist(S2E, rho), what is the meaning of rho

#1 
# Condition on observations.
        t_is_observed = isinstance(t, slice) or t < self.duration
        pyro.sample(
            "obs_{}".format(t),
            binomial_dist(S2E, rho),
            obs=self.data[t] if t_is_observed else None,
        )

what is the meaning of rho here? We’re observing new infections; logically, shouldn’t these new infections align with the E2I transition?

Thanks