Earlier versions of NPR 7150.2, NASA Software Engineering Requirements, assumed that readers would accept requirements marked by an "X" in Appendix D as required activities. Experience with the baseline version of this NPR, i.e., NPR 7150.2, showed that this assumption was not universally confirmed.
NPR 7150.2 was written to include the explicit statement in the requirement that full compliance with the requirement is necessary for those entries assigned an "X" in Appendix D.
The purpose of the Requirements Mapping Matrix in NPR 7150.2 is to essentially pre-tailor out requirements for less critical software systems, e.g., while Class A software has 111 invoked requirements for projects, Class H has only 16. The purpose of the "X" label is to clarify the intent of NPR 7150.2 and to preclude any alternate interpretation of invoked requirements.