https://www.thedrive.com/the-war-zone/29092/special-access-programs-and-the-pentagons-ecosystem-of-secrecy

…The early origins of Special Access Programs can be traced to March 22, 1940, when President Franklin D. Roosevelt signed Executive Order 8381, creating for the first time, three security designations for America's most crucial information—restricted, confidential, and secret.

In the ensuing years, various presidential executive orders have tweaked how the government designates classified information. The current levels of classification are confidential, secret, and top secret. The determining factor for how information is classified depends on how much damage an unauthorized disclosure would reasonably be expected to cause.

Top Secret: “Exceptionally grave damage to national security.”

Secret: “Serious damage to national security.”

Confidential: "Damage to national security.”

Important to note, established through the Atomic Energy Act of 1954, the Department of Energy (DOE) uses two differing equivalent levels of security clearance.

Q-Clearance: Equivalent to a Top-Secret level clearance.

L-Clearance: Equivalent to a Secret level clearance…

(Very long, but informative article on SAPs, such as Area 51)