An important aspect of systems is that each component can be considered as a subsystem. In the health centre appointments system, the 'computerised booking system' may be a complex system in its own right involving a number of computers networked together. Figure 2 shows this view of the system with 'computerised booking system' composed of two subsystems: 'network' and 'computers'. Of course, these subsystems may also be complex systems in their own right, composed of further subsystems.
A complex overall system can therefore be reduced to more manageable proportions by grouping components together and thinking about it in terms of subsystems. You can think of the subsystems as being 'nested' inside a system or another subsystem, a bit like Russian dolls. No single one of them is 'the doll'; each one fits inside the larger one. A system map can help by allowing us to focus on the particular systems and subsystems that we are interested in.