I used to think it meant “for your eyes only”, i.e.: Don’t show it to anyone else. But a year or two ago, I was reading a book in which was reprinted a document from either WWII or the early part of the Cold War (my memory is foggy) that had at the top both “for your eyes only” [followed by the name of the person the document was intended to be read by] and “eyes only”. This got me to thinking (always dangerous). I know that “top secret” isn’t the most secret sort of U.S. government document. Is “eyes only” one of the more-secret-than-top-secret designations? If so, is it supposed to mean that it is not to be copied (i.e.: Use only your eyes, not the Xerox machine or a camera)?
My understanding is that “eyes only” has nothing to do with classification.
I think “eyes only” means just that the document is to be opened and read by the addressee, not by a postal clerk, an administrative assistant, a spouse, etc. I have seen medical records marked “eyes only.”
The classification (i.e., confidential, secret, top secret, etc.) specifies how the document is to be handled (i.e., how it is to be secured, the minimum security clearance required for anyone who handles it, etc.)
“Eyes only” means no cameras, copiers, etc. In other words, you can look (if your clearance is high enough) but you can’t take it with you in any way–even as an image.
Related term is “ears only”.
It means “Do not fondle this document.”
I worked for a “special” group when I was in the Navy. While groups like the NSA and CIA may have different classifications, this is what I saw and learned through my job.
The “top secret” classification has three code words associated with it, each denoting still higher levels. The code words themselves were classified and may have changed since I was in. Ask me what those code words were and I shall refer you to a rather infamous line from “Top Gun” 
“Need to know” is an important tenet of intelligence. I was cleared for the highest level of TS information because I was a technician who had acess to all secure spaces and I might hear or see something highly classified. I did not have a need to know, however, so certain spaces were restricted at certain times to those not in the know.
“Eyes Only” is not a higher level of classification. Yeah is correct in saying that the item is to opened only by the addressee, but the addressee could be one person, a department, an entire command, etc.
I can’t agree enough with everything sandyr says, except to correct one item of spelling. It should have read:
“KneadToKnow is an important tenet of intelligence.” (emphasis mine).
And thanks, sandyr, for your endorsement.
:giggles:
:ducks:
:runs: