What does the notation (S) signify in classified documents?

The notation (S) in classified documents signifies "Secret" information. This classification level indicates that the information, if disclosed without proper authorization, could cause serious damage to national security. Understanding the significance of this designation is crucial for personnel handling classified information, as it helps determine the level of protection required for such material. This classification is part of the overarching system used to categorize information based on its sensitivity and potential impact on national security, ensuring that individuals with the appropriate security clearance handle it.