Which document is essential for properly marking a classified source?

The essential document for properly marking a classified source is a security classification guide. This guide provides the necessary instructions and criteria for determining how information should be classified, as well as the specific markings that need to be used. It outlines the classification levels, the rationale for classification decisions, and ensures that personnel are consistent in their application of security markings. This is crucial for maintaining the integrity of classified information and safeguarding national security.

In contrast, other documents serve different functions. A manual for data handling may provide general procedures for managing classified data but does not specify how to appropriately mark it. A non-disclosure agreement primarily focuses on obligations regarding the handling of classified information rather than the classification process itself. A user training certificate indicates that a person has completed training, but it does not provide the necessary guidance for marking classified sources. Therefore, the security classification guide stands out as the primary resource for ensuring correct and uniform marking of classified information.