Anything that's unclassified hasn't been sorted or organized into a certain category. If your symptoms can't be identified as a specific illness, your doctor may describe you as having an unclassified condition.
Science is all about classifying things, putting them into classes or groups based on common traits. Things that haven't been thoroughly studied by scientists remain unclassified, like creatures at the very bottom of the ocean or genes that remain mysterious. Another meaning of this adjective is "not secret," usually when we're talking about documents: "The unclassified pages of notes from the President's meeting were published by the newspapers."