What marking must be used for documents containing both classified and unclassified information?

When a document contains both classified and unclassified information, it is essential to mark the document according to the highest classification level present. This practice ensures that the entire document is treated with the necessary level of protection that matches the most sensitive information it contains. By marking it at the highest classification level, individuals handling the document are made aware of the potential risks associated with accessing or disclosing any part of the content, regardless of whether some of it is unclassified. This ensures compliance with security regulations and protects the integrity of the classified information.

In contrast, marking the document with only the lowest classification or unclassified marking could lead to inadvertent exposure of sensitive information, as individuals may not fully grasp the implications of handling the unmarked parts. Furthermore, a lack of any marking may create uncertainty about the document's security status, increasing the risk of mishandling. Therefore, using the highest classification level present is the standard and safest approach to facilitate proper safeguarding and dissemination of classified material.