While sounding counter-intuitive, a classification of "unclassified" simply means that no decision has been made. And if no decision on the actual classification has been made then it is possible that in the future that classification might be decided to be "secret". Which implies that if "unclassified" needs to be at least as strong as the highest explicit classification.
Aside: Fans of Gödel might be wondering that how can be talk about classifying something as being unclassified, which in turn is higher in some sense than the highest classification. Simply, "unclassified" is a label which is is attached to everything before the formal process of attaching on of the other labels is made.
Let's start with an example: if Alice writes a document then this document by default is unclassified. Typically that document's handling falls under the responsibility of Alice alone.
If Alice gives that document to Bob, then Bob must handle that document according to Alice's specific instructions.
By implication Alice has chosen a particular security classification. There are two choices:
- Either an explicit classification is given, eg: secret, confidential, public
- Or, no classification is given an Alice remains the authority for instructions on how to handle that document
In the latter case Alice's instructions may be tighter than the highest, explicit classification, which implies that unclassified is more restricting than, say, secret.
If Bob passes the document to Eve (or to the whole company by a reply-all) then we have a data breach. The document never implicitly becomes public through this means; though over time the document might become public knowledge but still remain officially secret. For example, if an employee of a company leaks future product specifications to the media, even though they are now effectively public, the employee (and others) who handled the data would still fall under whatever repercussions leaking secret or confidential data implies.
Still this is awkward to reconcile, so we need more structure here to understand what unclassified and the other classifications mean.
We must therefore apply to a notion of authority: all documents must have an owner - this is basic document handling. That owner
- Either assigns an explicit security classification, and all handlers of that document refer to the standard handling procedures for that security classification: referring to the security classifications standard as the authority
- Or, keeps the document as being unclassified and makes themselves the authority for rules on how to handle that document
The latter also comes with the implication that the owner of the document here is also responsible for ensuring that whatever handling rules are implied, these are consistent with the contents of the document. For example, if the document contains sensitive data then in our example, Alice is responsible for ensuring that the rules that come from her authority are as at least as strict as the highest implied security classification.
In summary, if a document or data-set is unclassified then the owner of that document is the authority deciding on what the handling rules are and that by default the rules must be at least* as strict has the highest explicit security category.
*In our classification we have the relationship:
Public < Confidential < Secret
with the statement above saying:
Public < Confidential < Secret <= Unclassified
As a final point, if Alice decides that her rules are weaker than say, confidential, but stronger than public, then it makes sense to take the next highest level as the explicit classification, ie: confidential. This way we establish the policy that all documents must eventually be explicitly classified.