In Data Flow Diagrams (DFDs), the flow of data is universally represented by a directed line, or an arrow. The arrowhead indicates the direction of data movement. This symbol connects other DFD components, such as external entities, processes, and data stores, illustrating the path data takes through a system. The arrow is labeled with the name of the data it carries. This notation is standard across different DFD methodologies, including both Gane & Sarson and Yourdon/DeMarco.

A. Square: A square (or rectangle) represents an external entity, which is a source or destination of data outside the system's boundary, not the flow of data itself.

B. Rectangle: Similar to a square, a rectangle is used to depict an external entity. A rounded rectangle is typically used to represent a process that transforms data.

D. Data Store: A data store, usually shown as two parallel lines or an open-ended rectangle, represents a repository where data is held for later use, not the movement of data.

1. Hoffer, J. A., George, J. F., & Valacich, J. S. (2011). Modern Systems Analysis and Design (6th ed.). Prentice Hall. Chapter 7, "Structuring System Process Requirements," Figure 7-3, p. 211, explicitly defines a data flow as "Data in motion, moving from one place in a system to another" and its symbol as an arrow.

2. University of Texas at Austin, McCombs School of Business. (n.d.). Data Flow Diagramming (DFD) Symbols. Retrieved from https://www.mccombs.utexas.edu/~/media/Files/MSB/Depts/IROM/MIS-Cases/Fall2012/dfdsymbols.pdf. This document shows both Gane & Sarson and Yourdon & DeMarco notations, where the "Data Flow" is represented by an arrow in both systems.

3. Yourdon, E. (1989). Modern Structured Analysis. Prentice-Hall. Chapter 9, "The Dataflow Diagram," pp. 137-140, defines the data flow as a "pipeline through which packets of information of known composition flow" and illustrates it with a directed arrow.