A context diagram is a top-level (also known as Level 0) data flow diagram. The book ticket context diagram only contains one process node that generalizes the function of the entire system in relationship to external entities. The zero level DFD of the book ticket is a basic overview of the whole ticket reservation system or process being analyzed or modeled by the accounting department. The context diagram is designed to be an at-a-glance view of either online or offline booking, where an agent from a travel agency categorizes and validates ticket reservations in order to prepare the tickets for the passengers. At the same time, passengers can also check and confirm the tickets from their own ends.