These relationships can be portrayed in a Product Flow Diagram (PFD).
Figure 2.3 gives an example. Note that the ‘flow’ in the diagram is assumed to be
from top to bottom and left to right. In the example in Figure 2.3, ‘overall system
specification’ is in an oval as it is used by the project but is not created by it. It
is often convenient to identify an overall product at the bottom of the diagram,
‘operational system’ ,say, into which all the other products feed.