A C4 system context diagram for any application in the sample ecosystems: the people who use it and the other systems it shares work with. Neighbours are calculated from the feature links, so two applications are connected when they deliver the same features. Use it when a system is being replaced, integrated or rationalized; go deeper (containers, components) only for the system being designed.
Sample data: applications and links come from the fictional Example State University ecosystems.
C4 notation: person shapes are users, the dark box is the system in focus, grey boxes are other software systems, dashed boxes are manual work. Each line says how many features the two share and who holds the record. A red line means both claim to be the system of record for some of the same features.
Structurizr DSL (.dsl): paste into the free editor at structurizr.com/dsl, or open with Structurizr Lite. It holds the people, systems and relationships plus a system context view; add containers inside the focus system when you start designing it.
C4-PlantUML (.puml): paste into plantuml.com/plantuml, or open in VS Code with the PlantUML extension. It pulls the C4 library from GitHub, so it needs an internet connection the first time.
Next level down: a container diagram shows the applications and data stores inside one system. Draw it only for the system being built or replaced, with the vendor’s architecture documents.