Sorted into classes or categories

Formally assigned by a government to one of several levels of sensitivity, usually (in English) top secret, secret, confidential, and, in some countries, restricted; thereby making disclosure to unauthorized persons illegal.

“We do not discuss specific interrogation techniques because they are classified information.”

Not meant to be disclosed by a person or organization.

To identify by or divide into classes; to categorize

“Should we classify "make up" as an idiom or as a phrasal verb?”

To declare something a secret, especially a government secret

“They decided to classify that information.”