SuDoc: The US Government created its own system to classify government publications called the Superintendent of Documents system or SuDoc. Call numbers contain a combination of letters, numbers, decimals and colons. Call numbers begin with letters.
Call Numbers: The initial letter in the SuDocs call number represents the government agency that produced the document. In the example below, the call number starts with A, for Dept. of Agriculture (A 93.56:IUS-5 S). A93 is for the Economic Research Service, which is under the Department of Agriculture. The number following the period indicates the series or type of publication, and the rest of the number, following the colon, identifies the individual publication in that series.
SuDoc classification system
A - Agriculture Dept.
AE - National Archives
C - Commerce Dept.
C 3. Census Bureau
D - Defense Dept.
E - Energy Dept.
ED - Education Dept.
GA - General Accounting Office
GS - General Services Admin.
HE - Health & Human Services Dept.
HH - Housing & Urban Devel. Dept.
I - Interior Dept.
I 19. US Geological Survey
J - Justice Dept.
JU - Judiciary
L - Labor Dept.
LC - Library of Congress
NAS- NASA
S - State Dept.
SI - Smithsonian Institution
T - Treasury Dept.
T 22. IRS
TD - Transportation Dept.
VA - Veterans Affairs Dept.
X,Y - Congress
Y 1. Congressional Reports
Y 4. Congressional Hearings