Superintendent of Documents Classification is a system of library classification developed in the office of the Superintendent of Documents of the United States Government Publishing Office (GPO) at the turn of the 20th century. The call numbers are based up on issuing agency / department. For example, A call numbers are publications of the Department of Agriculture, C is Commerce, D is Defense, and so on.
While most academic libraries use the Library of Congress (LC) Classification system to organize materials by subject/discipline, government resources use a completely different system.
The most important aspect to remember is that the first letter of the SUDOC number corresponds to the department or agency that issued the publication