I might be the only regular poster on AH who ever had a security clearance. Everyone with a high level clearance was limited to knowing only those secrets that directly pertained to his or her work. It was called "Compartmentalization". I had a very low rank, but Compartmentalization applied to everyone, no matter what rank, even the President. If a president wanted to visit Area 51, any clearance required to go there would have not been granted. It is likely that he could be briefed on some of what happens there, but the briefing would only be for him, and not the White House aides and press who might accompany him if he actually traveled there.
I had a TOP SECRET UMBRA clearance while in the US Navy until I was debriefed in 1975. That word UMBRA was itself highly classified, and it was followed by a few other code words, also highly classified. They stood for highly classified missions that I directly worked on. I knew plenty about radio and satellite communications of a certain nation that no longer exists today. It's language had an alphabet with 29 letters, unlike ours which has 26 letters. I knew absolutely nothing about our own radio and satellite communications. And of course, I knew nothing about anything else classified.
After getting out, I had a 5-year travel restriction, until 1980. No travel to any Communist nation, or other nation on a trouble spot list maintained by the Defense and State Departments. If I applied for a travel visa to any of those countries, my request would have been blocked. I guess, after 5 years passed, whatever I knew was no longer considered sensitive.
