The hierarchy is

• Federal laws are bills passed by the United States Congress and signed by the President such as The Federal Food, Drug, and Cosmetic Act (FD&C Act) of 1938. Individual laws are called acts or statutes.
• These Acts of Congress are arranged by subject into United States Code (USC) under one of 50 titles. The FD&C Act of 1938 and subsequent amending statutes are codified into Title 21 of the USC, beginning 21 USC 301.
• The executive departments and agencies of the government such as FDA have authority to make official rules and regulations that clarify and explain the United States Code, which are published as Code of Federal Regulations (CFR). These regulations carry the same force of law as the original statute/act/USC. The CFR is the codification of general and permanent rules.

Example of a hierarchy (here)

• FD&C Act Section 505A = STATUTE
• 21 USC Section 360aa - Drugs for rare diseases (here) = CODE
• 21 CFR Section 316 - Orphan Drugs (here) = RULES & REGULATIONS
• FDA Guidance documents - these are generally recommendations unless specified otherwise

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SOURCES

• What is the difference between the Federal Food, Drug, and Cosmetic Act (FD&C Act), FDA regulations, and FDA guidance? FDA Website. 28 March 2018
• United States Senate/Reference/Laws and Regulations
• Ask Civics 101: What Are The Differences Between Laws, Regulations, Ordinances, And Statutes. By Mitchell Scacchi. New Hampshire Public Radio. 6 August 6, 2021
• Reddit ELI5: What is the difference between the United States Code (USC) and the Code of Federal Regulations (CFR) and how do they relate to each other