Use data flow diagrams (context and level-0 diagrams) to describe information flow in a drug store's prescription processing system:

A patient presents her prescription to the system. The system checks if this prescription is new (a new prescription or a refill) by searching in its prescription database. If it is a new prescription, a new prescription record is inserted into the prescription database. The system then checks if the patient is a new patient by checking the patient database. If she is a new patient, a patient record is created in the patient database.

No matter whether the prescription is new or refill, (1) a label is created based on information retrived from the patient database and the prescription database; and (2) the system checks the inventory database to see if the durg is available.

If the drug is available, a transaction record is created and saved in the prescription database. If the drug is unavailable, a OOS information is shown to the patient.