In a data-flow diagram (DFD), what does a square symbol represent?

Introduction:
In structured analysis, a data-flow diagram (DFD) uses a small set of standard symbols so that analysts, developers, and business users share a common visual language. This question checks recognition of the square symbol and reinforces the meaning of each core DFD element.

Given Data / Assumptions:

• Context is a standard Yourdon or Gane–Sarson style DFD.
• Symbols include square/rectangle, circle/rounded-rectangle, open-ended bars, and arrows.
• Terminology: external entity, process, data store, data flow.

Concept / Approach:
A DFD shows how data moves and is transformed. The external entity is outside the system boundary and either supplies data to the system or consumes data produced by the system. It is drawn as a square or rectangle. Processes are circles. Data stores are open-ended bars. Data flows are arrows.

Step-by-Step Solution:
1) Identify the symbol in question: a square. 2) Map symbol to DFD legend: square = external entity. 3) Define external entity: a source or destination of data, outside the system boundary. 4) Conclude that the square denotes originator/receiver of data.

Verification / Alternative check:
Cross-check with any standard systems analysis text: the square or rectangle is consistently labeled as an external entity or terminator.

Why Other Options Are Wrong:
Option B: Arrows denote data flow, not squares.
Option C: Processes are circles or rounded rectangles, not squares.
Option D: Data stores are open-ended parallel lines, not squares.
Option E: DFDs do not include a dedicated “control” symbol as a square.

Common Pitfalls:
Confusing external entities with processes, since both can be drawn as boxes in other notations. Remember that in DFDs, the square is specifically external to the system.

Final Answer:
Defines a source or destination of system data (external entity)