Which document is the original source for categories of information that are eligible classification?

The original source for categories of information that are eligible for classification is Executive Order 13526, titled "Classified National Security Information."

This executive order, signed by President Barack Obama in 2009, defines the different levels of classification (Top Secret, Secret, Confidential) and outlines the types of information that can be classified. It also establishes procedures for handling classified information.

While there have been updates and revisions to the order over the years, Executive Order 13526 remains the foundational document for classifying information within the United States government.