A requirement specifies a capability or condition that must (or should) be satisfied. A requirement may specify a function that a system must perform or a performance condition a system must achieve. Use cases (which existed in UML) are effective for capturing the functional requirements, but not suitable for expressing non-functional requirements. The incorporation of text-based requirements into SysML effectively accommodates a broad range of requirements.
A Requirement Diagram is a new type of diagram specially used in SysML in which requirements and the relations are to be specified between them. A standard SysML requirement includes properties to specify its unique identifier and text requirement itself, additional properties such as verification status, priority, etc., can also be specified by the user.
Why We Need Requirement Diagram in SysML?
SysML includes a requirements modeling capability to provide a bridge between the text-based requirements that may be maintained in a requirements management tool and the system model. This capability is intended to significantly improve requirements management throughout the lifecycle of a system by enabling rigorous traceability between the text-based requirements and the model elements that represent the system analysis, design, implementation and test cases.