Earlier quoted context omitted.
Bear in mind that you have to have a "need to know" in addition to your clearance. We can speculate that there are also levels beyond top secret. In particular, eyes-only for the most sensitive stuff [1], and (what I have seen called but can't find a reference for) code-word secrecy for particular projects/operations. [1] Which is where the Bond film For Your Eyes Only gets its title.
The clearance levels that I can remember are: confidential secret top secret top secret ssbi -- "single scope background investigation" top secret sci -- "secured compartmentalized information" IIRC There are probably others above that that I don't remember. TS and TS/SSBI might be the same thing with different investigations. I'm not sure.
Arbitrarily high levels of clearance can probably be implemented as separate SCI authorizations.