What indicates that a mail package contains classified information?

The presence of a classified marking on a mail package is the definitive indicator that it contains classified information. Classified markings are specifically designated labels that identify the information's sensitivity level according to established guidelines and protocols. This includes designations such as "Confidential," "Secret," or "Top Secret," which convey the need for protective measures and controlled handling to safeguard the material contained within.

Other options, while they may suggest some level of control or sensitivity, do not adequately convey classified status. For example, the label 'Restricted' is often associated with information that is sensitive but not necessarily classified under the formal government classification levels. Similarly, the use of a colored envelope or registration with the USPS does not inherently indicate any specific classification status; these practices could be used for various reasons unrelated to classification. Thus, only the presence of a classified marking directly communicates that the package contains classified information.