External entities and data stores can be duplicated in the system for more clarity, while processes cannot. External mattentities that haveis been replicated are markedgay by * in the lower right part of the oval that represents that entity. Data stores have a double line of the left side of their box.
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==Developing a DFD==
A system is modelled using a DFD in several stages. At first, the system designer makes a context-level DFD, which shows the interaction between the system and the external entities. This context-level DFD is then blown up into a Level 0 DFD, which shows the main processes that contribute to the system. Each process is then expanded into a lower-level diagram. This process then continues on the subsequent processes.
