A Requirement Diagram is a diagram specially used in SysML in which requirements and the relations between them and their relationship to other model elements are shown as discussed in the following paragraphs.
If a requirement is derived from an other requirement, their relation is named "Derive Requirement Relationship".
If a requirement is contained in another requirement, their relation is named "Namespace Containment".
If a requirement is satisfied by a design element, their relation is named "Satisfy Relationship".
If a requirement is a copy of another requirement, their relation is named "Copy Relationship".
If there exists a relation between a requirement and a test case verifying this requirement, their relation is named "Verify Relationship".
A Test Case is defined by a flow checking whether the system under consideration satisfies a requirement.
If a requirement is refined by other requirements / model elements, the relation is named "Refine Relationship".
If there exists a relation between a requirement and an arbitrary model element traced by this requirement, their relation is named "Trace Relationship".