At the highest level, United States Code, Title 10, Sect 113 Secretary of Defense in (b), "The Secretary is the principal assistant to the President in all matters relating to the Department of Defense. Subject to the direction of the President and to this title and section 2 of the National Security Act of 1947 (50 U.S.C. 3002) he has authority, direction, and control over the Department of Defense."
The Secretary of Defense authorities were delegated to component heads, based on topic areas. DoDI 5025.01, found here on the DoD Issuances website, https://www.esd.whs.mil/Portals/54/Documents/DD/issuances/dodi/502501p.pdf?ver=2019-05-22-113758-133, details the authority over the issuance process to OCMO. In the instruction it notes that " Issuances will be used to codify DoD policy and guidance...". Thus, as issuances are DoD policy and are part of direction and control of the DoD, they should be viewed as mandatory.