Safe: Objects that are easily contained and safe. Euclid: Objects that are complex to contain and for which complete containment is not always guaranteed. Keter: Objects that are extremely difficult to contain and pose a high risk of containment breaches, potentially triggering K-Class End-of-the-World scenarios. Thaumiel: Objects that are useful for containment or have the potential to stop K-Class scenarios caused by Keter-class objects. Apollyon: Objects that are impossible to contain and pose a catastrophic threat.

Mobile Task Forces (MTF): Units that respond to and contain anomalies; each unit has a distinctive name. Groups of Interest (GoI): Groups other than the Foundation involved with anomalies; mostly hostile organizations.

Class A Personnel: Deemed vital to Foundation strategic operations; prohibited from contact with objects under any circumstances. Class B Personnel: Deemed essential to strategic operations; allowed contact only with objects confirmed to be safe. Class C Personnel: Allowed contact with most objects that are not extremely aggressive or dangerous. Class D Personnel: Expendable personnel used for controlling or experimenting on particularly dangerous objects; prohibited from contact with Class A and B personnel. D-Class are recruited from death row inmates. (Designated by a 4-digit number after 'D', e.g., D-1234). Field Agents: Responsible for information gathering during securing operations. Researchers (including Doctors): A subset of Class C to Class B personnel. O5 Council: The supreme command consisting of five Class A individuals. Personnel Class C and above are resilient and generally not intimidated by SCP objects. Amnestics: Drugs used to erase or rewrite temporary or long-term memories; graded from Class C (lowest), Class B (middle), to Class A (highest) depending on strength.