Each individual system itself is viewed as being logically composed of a succession of subsystems, each
corresponding to the intersection of the system with a layer. In other words, a layer is viewed as being logically
composed of subsystems of the same rank of all interconnected systems. Each subsystem is, in turn, viewed as being
made of one or several entities. In other words, each layer is made of entities, each of which belongs to one system.
Entities in the same layer are termed peer entities