The classes are on how hard are the objects to contain, there’s a simple analogy for this:
Safe: If you can put the entity in a box and it won’t ever escape, then it’s safe
Euclid: If you put the entity in a box and it might break out, then it’s Euclid
Keter: if you can put the entity in a box and it consistently can escape, then it’s keter
Thaumiel: If the entity is the box , then It’s thaumiel
Apollyon: If there was never a box that could contain this or could never be put in the box, it’s Apollyon