The level 1 diagram shows the main functional areas of the system under investigation. This level (level 1) shows all processes at the first level of numbering, data stores, external entities and the data flows between them. As with the context diagram, any system under investigation should be represented by only one level 1 diagram. So, level 1 DFD provides a high level view of the system that identifies the major processes and data stores.
The purpose of this level is to show the major and high-level processes of the system and their interrelation. A level-1 diagram must be balanced with its parent context level diagram It is surrounded by the outline of a process box that represents the boundaries of the system.