Data Flow Diagrams (DFDs) illustrate the way that the functionality within each Functional Area is organised into a hierarchy of High-level and Low-level Functions. They also show how the Functions are linked to each other, and to the Terminators and Data Stores through Data Flows. There is one DFD for each Functional Area, and within them, one DFD for each High-level Function. Each DFD is numbered and named. The first digit of the number identifies the Functional Area. Any second and subsequent digits correspond to the High-level Function that the DFD represents. The name of each DFD is either that of the Functional Area or the High-level Function that each DFD represents
The DFDs contain rectangular shapes, representing the Functions, and (sometimes) cylindrical shapes representing Data Stores. Data Flows are represented by lines with an arrow indicating the direction of the flow of data. The name of the Data Flow is written within the line. The sizes of the rectangles and cylindrical shapes vary from one DFD to another and have no special significance.