Which marking indicates that a document has significant national security implications?

The marking that indicates a document has significant national security implications is "Classified." This designation is used to protect information that, if disclosed without authorization, could harm national security. Classified materials are subjected to various levels of protection depending on the sensitivity and potential impact of the information; these levels often include Confidential, Secret, and Top Secret, each denoting increasing degrees of sensitivity.

In contrast, non-sensitive markings are used for information that does not require special protection and poses no risk to national security. Proprietary refers to information that is owned by a company or individual and may be subject to trade secret laws, but it does not directly relate to national security concerns. The term "Sensitive" can pertain to information that is not classified but still requires careful handling due to its implications, although it does not convey the same level of urgency or significance regarding national security as classified information. Thus, the use of "Classified" underscores the critical importance of safeguarding national security-related information.