Though the concept of subsystems and that of component systems are quite different, both viewpoints to a part of a system can be unified in some cases. In those cases, we can treat a subsystem and a component system, interchangeably. In this paper, we will focus on the condition for systems under which we can identify them. Firstly, we review mathematical foundations for the discussion. Secondly, we will investigate a mathematical structure for which a subalgebra and a quotient algebra can be identified. Finally, we adopt the discussion to general systems.