Classified means officially secret, with access limited by rank or clearance. Classified documents, a classified briefing, remains classified.
The levels in Britain and America run through restricted, confidential, secret and top secret, and clearance is granted per level and per subject. Declassified means released. Classified also means simply sorted into categories — classified by size — and in a newspaper the classified advertisements, or classifieds, are the small ones arranged by category. Note that classified information and a classified advertisement are the same word doing very different work.
1adjectiveclassifiedOfficially secret, with access limited by clearance.
“The documents were classified, so only senior officers could read them.”
2adjectivecategoriesSorted into categories.
“The classified advertisements ran to about six pages.”
classified is a state of restriction or sorting.
The word is class plus -ify, from Latin classis, a division of the Roman people. Something classified has been put into a class, and the secrecy sense comes from the classes of restriction rather than from any idea of hiding.
French
classifier
English
classify
English, today
classified
From classify + -ed. classify comes from French classifier. by surface analysis, class + -ify.
Etymology from Wiktionary, CC BY-SA