Developing a Traceability Matrix
How is the Traceability Matrix developed?
In the design document, if there is a Design description A, which can be traced back to the Requirement Specification A, implying that the design A takes care of Requirement A. Similarly in the test plan, Test Case A takes care of testing the Design A, which in turn takes care of Requirement A and so on.

There has to be references from Design document back to Requirement document, from Test plan back to Design document and so on.

Usually Unit test cases will have Traceability to Design Specification and System test cases /Acceptance Test cases will have Traceability to Requirement Specification. This helps to ensure that no requirement is left uncovered (either un-designed / un-tested).

Requirements Traceability enhances project control and quality. It is a process of documenting the links between user requirements for a system and the work products developed to implement and verify those requirements. It is a technique to support an objective of requirements management, to make certain that the application will meet end-users needs.