Authors in this paper propose a new approach for system classification, for the purpose of facilitating optimum maintenance decision making, in terms of the economic dependency of replacements among preventively replaceable items (PRI) within a system. This classification divides the existing systems into three categories; i.e., Independent-PRI Systems, Dependent-PRI Systems and Mixed-PRI Systems. Accordingly, the existing replacement policies are classified according to their math structures and compositions, and a multi-mode-replacement-policy idea is presented for modern large-scale industrial systems. Cost, reliability and spares requirement models under each policy are given. Conditions for the existence of each policy are also discussed. Finally, the application strategies of these policies in modern industries are investigated.