Code: A universally accepted, agreed upon, or established means of determining what something should be. Monitored for compliance by an authoritative agency that can be adopted by legal jurisdiction and made into a law.
Standard: provides requirements and specifications to ensure that products, such as valves, are fit for their purpose. A standard is not a mandatory specification but a reference which is applied or not by the manufacturer or asked by the customer
Recommended practice: an exact statement which is recognized as desirable in the interest of safety, regularity, or efficiency by an organization or customers. It provides for voluntary compliance