1.What is Data Flow Diagram DFD?
Data flow diagram is the diagram to visualize the flow of data from data source, data store and processes.

2.What are sub levels of Data Flow Diagram?
2.1 Context Diagram
-This is the first level of the DFD.
-In the context diagram, we just visualize the data flow between the data source and the system (0)
2.2 Level 0 Diagram
-In this diagram, the system will be divided into processes as 1.0 2.0 3.0 4.0 5.0
-In the level 0 diagram, we will visualize the data flow between the data sources, data stores and processes
2.3 level 1 Diagram
-In this diagram, a process will be divided into sub processes for example 1.0 will be divided into 1.1 1.2 .1.3
-In the level 1 diagram, we will visualize the data flow between the data sources, data stores and processes

Thus, we can have the next level diagram by continue to divide a process in a level to sub process in the next level.