Which of the following is NOT a Special Handling Marking?

The statement "Official Use Only (OUO)" is indeed not a Special Handling Marking, as it is primarily a category that indicates information is intended for official use and is not publicly available. It focuses on operational or administrative sensitivity rather than the specific handling requirements of the information itself.

In contrast, the other options represent designated special handling markings that indicate specific protocols for physical handling, distribution, or access control. "Restricted Data (RD)" pertains to sensitive nuclear-related information, while "Limited Distribution (LIMDIS)" is used for material that must be carefully controlled due to its subject matter or recipient limitations. "Encrypted for Transmission (EFTO)" typically signifies that information has been secured to prevent unauthorized access during communication, requiring particular handling and consideration to maintain security.

This differentiation clarifies the roles these markings play in information security practices, emphasizing the unique significance of each marking and helping to maintain appropriate levels of confidentiality and integrity for sensitive information.