What is the meaning of two parallel lines on a data flow diagram?

In a data flow diagram (DFD), two parallel lines represent a data store. This symbol is used to illustrate where data is held or stored within a system. Data stores are integral components of a DFD as they indicate persistent storage that allows data to be retrieved and used by processes within the system over time. By signifying a location where data resides, this symbol helps stakeholders understand how information flows through the system, what data is retained, and how it can be accessed by different processes and external entities.

Understanding this concept is crucial when analyzing data flow diagrams, as it allows for a clearer representation of how data moves through various stages in a system and enables effective communication among team members engaged in system design or analysis.