Earlier quoted context omitted.
Is "secret" not one of the classifications for internal documents? "top secret", "secret", "confidential", "restricted". While this classification is usually used to classify access to documents of the state, it could also apply to internal documents in large organizations.
I'm not aware of any private sector companies that have more than 2-3 formal "classification" markings, and they're usually more like "internal use only" and "confidential" rather than "secret". For particularly sensitive documents there would of course be additional restrictions, but I'd be surprised if there were companies using the marking "top secret" for that purpose...
This list is from a blog I wrote at https://rietta.com/blog/commercial-information-classificatio.... It was adapted from something I originally saw while a student at Georgia Tech. I've consulted with companies that separately developed a substantially similar 5 point list.