Which of the following is NOT considered a source for classification?

The rationale for identifying public domain information as not a source for classification lies in the fundamental principle that classified information is meant to be restricted from public access due to its sensitive nature. Information that is in the public domain is readily available to anyone and does not require protection from unauthorized disclosure. Therefore, it cannot substantively be classified, as classification is specifically reserved for information that, if disclosed, could harm national security or other sensitive interests.

In contrast, military operations, foreign government information, and weapons of mass destruction all encompass data that can indeed be sensitive and require a classification level to safeguard against potential threats or harm if such information were to be made publicly available. These categories inherently contain potential security risks, which prompt the need for careful classification to protect national security interests.