As others stated, the whole mystique around clearance levels is mostly something that is perpetuated by popular culture. There’s two basic rules when dealing with classified information: Is that person cleared to see it? Does that person have a need to know? That’s it. The classification is just about how dangerous it is, and I don’t remember the specific wordings, but it’s something like Serious, Grave, Extremely Grave. Really, what’s beyond that, except maybe instant annihilation? Stuff like battle plans and troop movements are the type of thing that will be TS.
The part that really matters is need to know. I work with computers, so I had particular need to know related to computer stuff like IP addresses, various security stuff, and I might happen to come across certain information. That said, I didn’t have a need to know, say, specifics about certain investigations, negotiations, boat/troop locations, or even active ongoing activities. Hell, there was one specific case that was computer related, but unless someone was specifically included in that briefing, they didn’t have need to know and I couldn’t say anything other than I was doing work regarding a particular operation, even to someone who had the same clearance and generally had the same types of work duties.
And, really, that’s the point. Classified information is compartmentalized to minimize damage. I may, theoretically, have potential access to a huge amount of information, but if I don’t need to know it, I don’t, so I can’t reveal it even by accident. Similarly, if I’m working on one part and someone else on another, one might really need both parts to REALLY cause a problem, but if only one of us is compromised, it’s much more manageable.
And it’s this latter part where the mysterious parts come in. And, generally, one can’t even say what part their working on because that helps people figure out who they need to compromise. That’s why you get vague responses.