DFD Symbols

1. A square defines a source (originator') or destination of system data.

2. An arrow identifies data flow-data in motion. It is a pipeline through which information flows.

3. A circle or a "bubble" (some people use an oval bubble) represents a process that transforms incoming data flow(s) into outgoing data flow(s).

4. An open rectangle is a data store-data. at rest or a temporary repository of data.

Note that a DFD describes what data flow (logical) rather than how 'hey are processed, so it does not depend on hardware , software, data  structure or file  organization. The key question that we are trying to answer is: What major transformations must occur for input to be correctly transformed into output?

Let’s expand process orders to elaborate on the logical functions of the system. First, incoming orders are checked for correct book titles, authors' names, and other information and then batched with other book orders from the same bookstore to determine how many copies can be shipped through the warehouse. Also, the credit status of each bookstore is checked before shipment is authorized. Each shipment has a shipping notice detailing the kind and number of books shipped. This is compared to the original order received (by mail or phone) to ascertain its accuracy. The details of the order are normally available in a special file or a data store, called "bookstore orders." 

Following order verification and credit check, a clerk batches the order by assembling all the book titles ordered by the bookstore. The batched order is sent to the warehouse with authorization to pack and ship the books to the customer.

Further expansion of the DFD focuses on the steps taken in billing the bookstore.. As you can tell by now, each process summarizes a lot of information and can be exploded into several lower-level, detailed DFDs. This is often necessary to make sure that a complete documentation of the "data flow is available for' future reference.