The three point object classification system currently employed by the Foundation is based on several factors, but primarily pertains to the amount of effort by the Foundation that is required to maintain security and containment, with containment being defined as "full suppression of the object's active threat to humanity or Foundation goals". In most but not all cases, this is an indicator of how dangerous the SCP object in question will be to personnel interacting with it in a containment or research scenario.
Addendum SEK-1-a: Note that the classifications apply only to current containment procedures; if an object's containment procedures change, its classification may change to reflect its new status.
Safe (easy to contain): The Safe classification is used to indicate that while an item is worthy of containment it is no longer a threat while contained, and can be disregarded so long as it remains in containment. So long as knowledge of the object is suppressed and non-Foundation people or entities are not aware of or able to access the object or its effects, a Safe class object requires the minimal level of effort from security personnel.
Euclid (moderately difficult to contain): The Euclid classification indicates that while an object can be contained indefinitely with moderate effort, it exhibits tendencies or effects which require the constant attention of personnel or increased fluidity in its containment procedures. Whether these tendancies spring from unpredictability, capacity for violence, or circumstances requiring nonstandard maintainance, Euclid class objects must be monitored regularly by security personnel.
Keter (difficult or impossible to contain): The Keter classification is applied to SCP objects whose effects or capabilities require constant effort to maintain containment, or whose full containment is not possible due to limitations in Foundation resources or capabilities. Keter class SCP objects pose the greatest security threat, and care should be taken by all personnel involved with their containment.