A primary Object Class that describes an item or entity that either cannot be reliably contained or otherwise poses an inimical threat to humanity. Keter-class objects and entities are highly dangerous and have extremely complex containment procedures.
A primary Object Class that describes an item or entity that is either unpredictable or otherwise insufficiently well-understood as to be reliably contained. Many consider this the "default" classification for new objects until such time that they are better understood or determined to be more dangerous than previously thought. Many consider any sentient and/or sapient entity to be automatically Euclid rather than Safe due to the fact that any entity with a mind of its own can always choose to stop cooperating with us.
A primary Object Class that describes an item that is sufficiently well-understood that it can be reliably contained with little to no risk. As famously stated, a Safe object can still very much be dangerous; a thermonuclear warhead in real life would be considered Safe as it cannot detonate unless multiple safety interlocks are removed.