The U.S. Federal Government and other governing bodies issue a multitude of directives, instructions, specifications and standards for their constituencies to follow; each having a different intention.
Looking for a specific industry or military standard? Check out the Standards & Handbooks tab on the Systems Engineering Subject Guide.
Directives: broad policy documents containing what is required by legislation, the President, or the Secretary of Defense to initiate, govern, or regulate actions or conduct. (Washington Headquarters Services)
Instructions: issuance that implements the policy, or prescribes the manner or a specific plan or action for carrying out the policy, operating a program or activity, and assigning responsibilities. (Washington Headquarters Services)
Specifications: outline requirements for specific items or services. (Yale University)
Standards: is a Publication of technical directions and regulatory material. (Washington Headquarters Services)