- To most, there are just Safe, Euclid, Keter. But there are 5 official classes. Safe, Euclid, Keter, Thaumiel, and Apollyon.
- As a note, any SCP that's autonomous, sentient and/or sapient is generally classified as Euclid, due to the inherent unpredictability of an object that can act or think on its own.
- Also note that the classes do not define how dangerous the anomaly is, but rather how difficult or easy it is to contain it
Class Safe (Easiest to contain)
Safe-class SCPs are anomalies that are easily and safely contained. This is often due to the fact that the Foundation has researched the SCP well enough that containment does not require significant resources or that the anomalies require a specific and conscious activation or trigger. Classifying an SCP as Safe, however, does not mean that handling or activating it does not pose a threat.
Class Euclid (Predictable)
Euclid-class SCPs are anomalies that require more resources to contain completely or where containment isn't always reliable. Usually this is because the SCP is insufficiently understood or inherently unpredictable. Euclid is the Object Class with the greatest scope, and it's usually a safe bet that an SCP will be this class if it doesn't easily fall into any of the other standard Object Classes. Also, most SCPs generally fall into this class.
Class Keter (Most difficult to contain)
Keter-class SCPs are anomalies that are exceedingly difficult to contain consistently or reliably, with containment procedures often being extensive and complex. The Foundation often can't contain these SCPs well due to not having a solid understanding of the anomaly, or lacking the technology to properly contain or counter it. A Keter SCP does not mean the SCP is dangerous, just that it is simply very difficult or costly to contain.
Thaumiel-class SCPs are anomalies that the Foundation uses to contain or counteract other SCPs or anomalous phenomena. Even the mere existence of Thaumiel-class objects is classified at the highest levels of the Foundation and their locations, functions, and current status are known to few Foundation personnel outside of the O5 Council.
Class Apollyon (Secondary Class)
Used for SCPs that are impossible to contain, or are about to irrevocably break containment. An SCP with this class will inevitably end the world. The classification was first used in SCP-927 on the Editthis wiki in 2008; it essentially meant that the SCP was a particularly dangerous keter. The original iteration of the Apollyon class did not appear in any additional SCPs, and was ultimately removed from SCP-927 by September of 2008. The class did not appear again until 2014, when it was used in SCP-2317. Dr. Clef, the author of 2317, did not intend for Apollyon to be a serious object class. Rather, the class was meant to indicate that the SCP article was abnormal, and that the documentation was something hidden even from most of the SCP Foundation. Dr. Clef came to regret re-creating the Apollyon class, and removed it from SCP-2317 in March of 2018. Despite Clef's misgivings, Apollyon has had a heavy influence on the SCP writing community. The class is used in SCP-3999 and S.D. Locke’s Proposal (an entry in the SCP-001 hub), both of which were created for the SCP-3000 contest. The class is also referenced in SCP-3148 (A2032), SCP-3799, SCP-3301 and SCP-3557. Following the removal of Apollyon from 2317, the class was used in SCP-4005 and SCP-4057 (both of which were SCP-4000 entries), Jim North's Proposal (an SCP-001 proposal), SCP-4023, SCP-4780, Meta Ike Proposal (an 001 proposal), SCP-4547, SCP-010-INT and SCP-5618.
During the SCP-5000 contest of January-February 2020, five additional Apollyon SCPs were posted. These SCPs were 5000:Evolution Revolution (since deleted), SCP-5500, SCP-5678, SCP-5055, and SCP-5321.
Following the 5000 contest, the class was used in SCP-5939, SCP-5092, SCP-5799, SCP-5391, SCP-3955, McDoctorate's Proposal (main article), SCP-4273, SCP-5157, and SCP-5956 (main article).
There are other classes, which are listed below. But are only used in other sites.
Class Explained (Secondary Class)
Explained SCPs are commonly articles about anomalies that are completely and fully understood to the point where their effects are now explainable by mainstream science or phenomena that have been debunked or falsely mistaken as an anomaly.
This class is used as a catch-all for SCPs - if an object is not Safe, Euclid or Keter, it is classed as Esoteric, in addition to whatever the secondary class is. This class was introduced with the Anomaly Classification System (ACS). Thaumiel and Apollyon are considered secondary classes under ACS, and so any objects that are Thaumiel or Apollyon are also considered Esoteric Class.
A classification for SCPs that were broken apart into components that are themselves anomalous, and that must be kept apart from each other. The class was first used by Twisted Gears-Kaktus Proposal, an entry in the SCP-001 hub. It is also referenced by SCP-3301 and SCP-4882. The class would later be used in SCP-5241 and SCP-5238.
Neutralized SCP's are no longer anomalous, either being intentionally/accidentally destroyed or deactivated. Usually meaning that they have died.
Class 53 (Secondary Class)
An SCP about which nothing is known at all, but has not proven to be dangerous yet either. This class is mostly referred to Scantron's Proposal SCP-001
A classification for SCPs that should not be contained (even though they are capable of being contained) due to the detrimental effects of containment.
The class originated in I.H.Pickman's Proposal, an 001 Proposal. There, the class indicated that containment would have a disastrous impact on the continued survival of humanity. More colloquially, the Archon class originally meant the SCP "will literally break the world if you contain it." The class was later used in SCP-3455 (where it referred to SCPs that are not contained because of the damaging effects of doing so- the more widely used definition and the one I use above), SCP-4200 (an entry in the SCP-4000 contest), SCP-1661 (replacing the SCP's former Euclid classification), SCP-3310 (replacing the SCP's former Hiemal class), SCP-4043, SCP-4494, SCP-4223, SCP-4778, SCP-4634, SCP-5015 (an SCP-5000 entry), SCP-5009 (also a 5000 entry), SCP-4614,SCP-5122, SCP-5520 and SCP-5644.
Anatta (Secondary Class)
An object capable of causing a YK-End-of-the-Universe Scenario, or altering the universe in a serious, potentially detrimental manner through non-improbable means. - Anatta is a buddhist concept meaning "non-soul" or, "not-self". It's the idea that all things perceived by the senses, including emotions, are not really "I" or "mine", and therefore, one should not cling to them. It's one of the three marks of existence in buddhism. If a person lost these characteristics, they would cease to exist. Just like the universe in a YK scenario.