When sending Secret information with a letter of transmittal, what special markings must be applied?

The correct response highlights the importance of specifying the handling instructions for Secret information when it is transmitted. When classified information is sent along with a letter of transmittal, it is crucial to indicate that the letter itself is classified. By stating "the letter is Secret when separated from classified enclosures," the sender ensures that the reader understands the classification level of the letter in relation to its content.

This marking serves a dual purpose: it protects the integrity of the classified information while also guiding the recipient on how to manage the document appropriately. It ensures that the letter does not fall into the hands of unauthorized individuals or gets mishandled simply because it appears to be a standard letter when separated from its enclosures. Such clarity in communication is vital in maintaining the security protocols surrounding classified materials.