A requirements traceability matrix (RTM) is a document that describes the relationship between customer requirements and test artifacts, especially test cases. The primary purpose of an RTM is to ensure that there are test cases for all requirements.
Every industry that develops software or manufactures hardware can use requirement traceability. But it's essential for quality and safety-critical enterprises to prove compliance.
There is no standard format of RTM; every organization uses its own. However, no matter which form you use, you ensure that it has bidirectional traceability. Bidirectional traceability is the ability to trace forward (e.g., from a requirement to its test cases) and backward (e.g., from a test case to its requirement).
Here is a sample RTM:
As you can see, you can trace the test cases based on requirements and vice-versa. For example, you can see that test cases for the ability to reset password has not yet been written.