Final answer:
Data Flow Diagrams (DFDs) should name processes and data stores based on data inflows or outflows, ensure data flows move in only one direction, and that flows originate from or lead to a process, data store, or an external entity. However, showing system start and stop points in a DDF is not a standard guideline.
Step-by-step explanation:
When preparing a Data Flow Diagram (DFD), there are specific guidelines to follow. First, Processes and Data Stores typically should be named based on the data inflows or outflows they receive or produce. This naming convention helps to clarify their function within the system. Secondly, it's correct that Data Flows should only move in one direction to maintain the clarity of the diagram and to accurately represent the flow of information. Lastly, all data flows ought to originate from and lead to a process, a Data Store, or an external source or destination. These guidelines help in creating a clear and readable DFD that accurately depicts the system's data processing.
However, the assertion that you should always show how the system starts and stops in a DFD is not a typical guideline. DFDs are primarily concerned with the flow of data rather than control flow, thus they do not typically illustrate start and stop points.