Before revision:  In most models, R is modeled as a ``universal " constant for the virus across outbreak clusters and individuals -- effectively averaging the effect of the inherent variability of the transmission process due to varying  population densities, demographics, temporal factors, etc .
After revision:  In most models, R is modeled as a ``universal " constant for the virus across outbreak clusters and individuals -- effectively averaging the effect of the inherent variability of the transmission process due to varying individual contact rates, population densities, demographics, temporal factors, etc .