The IDEF0 Context diagram is the starting node tree diagram. It describes the system you are modeling in its most complete and abbreviated form.

By IDEF specifications, the Context (or A-0) diagram contains one Function/Activity symbol, describing your model's top level function (with statements of purpose and viewpoint), and Input, Control, Output, and Mechanism (ICOM) arrows describing the system's inputs, controls, outputs, and mechanisms.

The IDEF0 Context diagram has two symbols available to the user - a symbol representing the model (a simple rectangle), and a symbol for drawing the function/activity on the diagram (a rectangle with an "M" in it). The model symbol is automatically added to the diagram when it is created - if you denote it as a context diagram.