Unlike most print resources in the library, which are classified with the Library of Congress (LC) call number system, U.S. federal government publications are arranged by the Superintendent of Documents (SuDocs) call number system. In the SuDocs system, publications are organized by their issuing agency. This differs from the LC system, in which items are organized according to their subject.

Each SuDoc call number begins with a letter or letters that indicate the agency/department that issued the publication. Some of these letters are straightforward (A for the Department of Agriculture, C for Commerce, D for Defense), while others are not (e.g., X and Y are used for various Congressional publications). Within an item's SuDoc number, you will also find which office within the agency/department is responsible for the publication, what category of publication it is (annual report, directory, bulletin, etc.), and information about the individual publication (its year, issue, edition, etc., depending on publication type).

A very basic break-down of a SuDocs number:

I 49.10:999 
First letter indicates issuing department (I=Department of the Interior)
First whole number before the dot indicates a subordinate office within that department (49=Fish & Wildlife Service)
First whole number after the dot indicates a category of publication, series, or serial title (10=directories)
Numbers or letters and numbers after the colon represent individual publications, years, issues or editions (pub date=1999)
 
A Note About SuDoc Numbers: 

SuDocs numbers do not read as a decimal, the way LC numbers do. Instead, the numbers before and after the dot are whole numbers. So, while D 19.18 would come before D 19.3 in LC order (because it reads as 19.one eight), it comes after D 19.3 in SuDocs order (because it reads as 19.eighteen).