Dalva:
Be the first on your block to know what data flow diagrams really are. The vast majority of BA's are clueless. Have great fun with this knowledge at parties!
First, data flow diagrams do not have to be used for computer system behavioral requirements. A system can be manual, automated, or combo thereof. DFD's can be used to specify as-is or to-be behavioral requirements for all three. The data storage symbol, for example, simply means data at rest - not a computer file. It can represent an 'in" basket on someones desk just as easily as a relational database table. You will find that all of this is hugely important to understanding DFD's, because those people who have it wrong are like totally not focused on what is right.
Now in specifying system behavior, there is behavior at a high level of abstraction (i.e., at the big picture level) and behavior at the lower levels (i.e., at the more detailed level). What is critical to know - and very, very people are aware of this - is that system behavior at the higher levels of abstraction is non-sequential (i.e., has no set sequence in which operations occur and no defined flow of control). Data flow diagrams are for capturing this " bigger picture". The processes on a data flow diagram are non sequential in that they can "fire" in any order, as long as the necessary inputs to the process (data flows) are present.
Concept: Business systems are complex. Handling complexity requires effective decomposition. Effective decomposition means working to various levels of abstraction. Working to higher levels of abstraction requires data flow diagrams. Got it?
Conversly, at the detail level, there are sequence dependent diagrams (like computer programming logical flow charts and BPMN). These document a defined order in which behaviors happen, utilizing flow of control (if-then-go-to) to branch the flow as required. Logiclly, DFD's and sequence dependent diagrams are used in com bination to show the entire picture.
For larger scale efforts, DFD's drill down multiple levels (parent process, child process, grandchild great grandchild) untill the reach bottom level sequential diagrams
Tony