Which category includes checking if the system behaves as expected under load?

The category that includes checking if the system behaves as expected under load is Non-Functional Testing. This type of testing focuses on aspects of the software that are not related to specific behaviors or functions of the system. Instead, it examines the system's performance characteristics, usability, reliability, and other quality attributes.

Load testing, which is specifically a subset of Non-Functional Testing, assesses how the system performs under a specific expected load. This testing is vital to ensure that the system can handle the anticipated number of concurrent users or transactions without performance degradation, ensuring that it meets the necessary performance criteria set by stakeholders.

Understanding the distinction between functional and non-functional testing is key. Functional Testing verifies that the system performs its intended functions correctly, while Non-Functional Testing emphasizes how well the system performs those functions under various conditions, such as high load. This makes Non-Functional Testing crucial for evaluating the overall user experience and satisfaction with the system in real-world scenarios.