What special markings must be applied to a letter of transmittal that includes background information for classified content?

A notice indicating that "the letter is Unclassified when separated from classified enclosures" is essential for providing clarity regarding the status of the document. This special marking ensures that anyone handling the letter understands that while it may contain background information related to classified content, the letter itself does not bear a classified status when it is detached from any enclosures that may be classified. This reduces the risk of misclassifying the entire document and promotes proper handling and dissemination practices for sensitive information.

In contrast, including a warning indicating that the letter is confidential does not address the classification status clearly when considering the context of separation from classified materials. A signature of a senior officer does not pertain to the classification markings required for a document itself. Similarly, a classification marking indicating the highest level of confidentiality would inaccurately categorize the letter as classified when it can be unclassified in a certain context, which the notice explicitly rectifies.