The system context diagram describes the top-level collaboration showing the system (modeled as a top-level subsystem), its interfaces and its relationships with its actors, including the external I/O entities that flow between actor and system.

Main information captured in the system context diagram:

• The interfaces to be realized by the system (in terms of the operations the systems provides, and the associated protocols supported, the state variables and stores that the system realizes, and attributes).
• The I/O entities that flow between the system and its actors.
• The interfaces required by the system (to be realized by the actors which interact with the system) for correct performance. Often, if the actor represents an existing system with which the system must communicate, these required interfaces simply reflect constraints imposed by that other system.