A person I know who did work in defense contracting described the difference between defense confidential and corporate confidential for a wind tunnel in this way:
defense confidential: the most important thing was to know if a breach occurred. Put a video camera on the door, put a piece of tape on the door, some how make sure that if someone came in the door, one knew it happened.
corporate confidential: the breach cannot occur. Lock the door, setup access controls, post a security guard checking ids
I'm sure that's not the whole story, but it was interesting to see hear the distinction b/w different approaches to secrets.