For example: an officer is theoritically allowed to read a report if he ask, but he isn't allowed to know about the existence or the secret level of that report.
Can the secret level of an infomation be lower than the infomation about its secret level? What specific situation can make it so? What infomation about infomation maybe more important than the infomation itself?
There are times when some of the meta-information in a security report is more important than the report data itself.
For example, the Allies went to great lengths to disguise the fact that they had broken the German Enigma machines. For the war efforts, knowing whether or not Enigma was hacked was far more important than any one message, and probably more important than entire days' worth of messages.
Or in today's terms, many articles have pointed out that one of the concerns over Trump's alleged leak of code-word level secrets to Russia isn't so much the secrets he gave them, but what those secrets tell Russia about how those secrets were collected and by whom.
And a third example of meta-data vs. data is the NSA wiretap exposed by Snow. The system, as explained by his leaks through various media sources, provided details such as who contacted whom, when, how often, by what means, and such. This lets their system build a web of interconnections that hints at who might be working with whom or who knows whom. All of those links are technically metadata.
There is a legal ruling in the UK called a Superinjuction 1
An example would be a footballer caught in a scandal; in this case the metadata is the fact that a court case is going on, which is deemed more important than the actual contents of the case. Therefore even the existence of the case cannot be reported on. It makes some sort of sense, because people tend to believe that something has happened as soon as a court case starts.
(This is highly dubious from a free speech perspective, obviously)
The only situation I can imagine where it's okay to inform others of the content of a message and not tell them about the message would be in the journalism industry. Specifically, a journalist trying to provide information to his/her readers while also protecting a source. (Also, from Andrew's answer, courts in the UK.)
Journalist: "Sources say the information was classified at the highest level."
Person: "Who are your sources?"
Journalist: "I can't tell you that."
With classification levels, it's the content of the information that is being protected. Any system that produces information generates that information at the highest classification level available to the system. A person may be allowed to know the information exists, but without both a Need to Know and the appropriate clearance, that person isn't allowed to see the information.
Of course, this being worldbuilding, you can always define your own classification system with the security level you have in mind.
One possible approach to hide information is to dilute it among a huge amount of false information.
Imagine for example of having the schematic of a new weapon: instead of concealing it, one can put it in a folder with thousands (or more) of other similar but wrong schematics.
The metainformation on which schematic is the correct one would then be more important than a single schematic.
Something similar was done in WW2: when the German occupied Netherlands and tried to use Philips factory to manufacture for their equipement, the management, instead of hiding the schematics simply corrupted them making them unusable for producing functioning equipments, and kept that information secret.
