Context diagrams represent a project’s product boundaries or “scope”. They model the product at a high level, in its “done” context. They do this by representing only the entities that interact with the product in that context. Thus, they do not represent the parts of the product or its behavior over time. As high level models of done products, they are very useful at a strategic level. They complement product visions, stories, or value propositions, customer segmentation, customer personae, etc. Also, context diagrams can be progressively elaborated into more detailed structural and/or dynamic models, including use cases, feature roadmaps, and/or system events.

Here is a fine 4-minute video on the context diagram construct.