On the one hand, the TRD defines test performance, test conditions, diagnostic requirements, and support equipment to fault locate, align, and verify proper operation of an item under test. It was used in the preparation of test packages (e.g., storage medium, manuals, and interface items, etc.) or test procedures for test equipment.  The TRD is no longer an active / current requirement and is replaced with more relevant artifacts such as a Test Plan for reliability and maintainability.

On the other hand, a test plan document outlines the planning for the entire test process. It has the guidelines for the testing process such as approach, testing tasks, environment needs, resource requirements, schedule, and constraints.  More specifically, the Reliability and Maintainability (R&M) Test Plan describes R&M tests and demonstrations at the equipment, subsystem and system levels (including Government developmental and operational tests), their purpose, and schedule. It identifies items to be tested, the test equipment and support requirements, the test conditions to be imposed, the parameters to be measured, and the criteria against which the test results will be measured. This document will be used by the procuring activity for review, approval, and subsequent surveillance and evaluation of the contractor’s R&M test and demonstration program.  Data Item Description (DID) DI-SESS-81585C “Reliability and Maintainability Test Plan” is germane. The aforementioned DID should be located through use of this https://quicksearch.dla.mil/Transient/BEC120AE3B1545EB8842EF4970B81356.pdf hyperlink.