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

# Condition on observations.
        t_is_observed = isinstance(t, slice) or t < self.duration
            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?