4.16. The performance of a geological disposal system is dependent on different physical components and other features having different safety functions, the importance of which may vary over different time periods. To meet the requirement for multiple safety functions, it is necessary for the safety case to explain and justify the functions provided by each physical component and other features and indicate the time periods over which they are expected to perform. It is also necessary for the safety case to identify the complementary safety functions that will be effective if a physical component or other safety function does not fully perform.