My Dark Materials
...the Classification Hierarchy (at least during my time as a spook) was:
Confidential
Secret
Top Secret
SCI (Special Compartmentalized Information)
Now, who decides what goes in which category, what determines the category boundaries, and what (if any) are the sub-categories), I do not know.
At least, that's how it was 30 years ago. I still have the receipts I was given when I returned all my classified materials. I was warned to keep them for the rest of my life.