Rules For Data Flow Diagram:

1. Each process must have a minimum of one data flow going into it and one data flow leaving it.
2. Each data store must have at least one data flow going into it and one data flow leaving it.
3. A data flow out of a process should have some relevance to one or more of the data flows into a process.
4. Data stored in a system must go through a process.
5. Filing systems within an organisation cannot logically communicate with one another unless there is a process involved.
6. All processes in DFD must be linked to either another process or a data store.