A level 0 DFD denotes the flow of data through a system. A level 0 DFD template usually begins with a context diagram, emphasizing the way data interacts with external entities. DFD Level 0 is often referred to as a Context Diagram. It provides a high-level perspective of the entire system or process being evaluated or modeled. It is intended to be a high-level overview of the system, displaying the system as a single high-level process with its relationships to external entities.