What type of requirement is characterized as "what a system must do"?

The designation of "what a system must do" clearly aligns with the definition of a functional requirement. Functional requirements specify the specific behaviors, functions, or capabilities that a system must exhibit in order to meet the needs of its users. They describe actions, tasks, and features that the product must support, focusing on how the system will operate and what specific outcomes it will produce.

For instance, a functional requirement could include specifications such as the ability to process transactions, generate reports, or validate user inputs. These requirements are essential for ensuring that the end product effectively meets the desired specifications and user expectations within the context of its intended use.

In contrast, non-functional requirements pertain to how a system performs its functions—covering aspects such as performance, security, or usability—rather than specifying what the functions themselves are. Technical requirements focus more on the underlying technology and environment necessary for operation, while quality requirements relate to characteristics such as reliability and maintainability. Therefore, the term "what a system must do" specifically refers to the core functionalities, making functional requirements the correct choice.