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

In the context of data flow diagrams (DFDs), two parallel lines represent a data store. A data store is a repository where data can be stored for future use in the system. This symbol indicates that data is saved for purposes such as retrieval or processing at a later time. The use of parallel lines helps differentiate a data store from other components like processes or external entities.

Each component in a data flow diagram serves a specific purpose. Processes, for example, are typically represented by circles or rounded rectangles, indicating actions or transformations being applied to the data. External entities are depicted as rectangles, showing sources or destinations of data that interact with the system. Data transitions, on the other hand, illustrate how data moves between these components, often represented with arrows.

Understanding the symbolism in a data flow diagram is crucial for accurately modeling and analyzing data flow within a system, making the identification of data stores particularly important for effective system design and documentation.