9. The number of packages, overpacks and freight containers containing fissile material stored in transit in any one storage area must be so limited that the total sum of the criticality safety indexes in any group of such packages, overpacks or freight containers does not exceed 50. Groups of such packages, overpacks and freight containers must be stored so as to maintain a spacing of at least 6 m from other groups of such packages, overpacks or freight containers.