System tests run against a deployed enterprise application. The application contains the same code, configuration, and runtime as in production. The test cases use external communication, such as HTTP, to initiate use cases. They verify that the overall outcome, such as HTTP responses, database state, or communication with external systems, matches the expectations of the application.

System tests answer the question what to test with: the application that runs in the same way as in production, excluding external concerns, accessed using its regular interfaces. External concerns will be simulated, ensuring predictable tests and enabling verification of the communication. It depends on the scenario whether a used database is seen as part of the application and used similarly, or mocked away as well.