The government publications collection is shelved by SUDOC (Superintendent of Documents) classification number. The basis of this system is to group together Government Publications by author, author meaning the department, commission, or agency that issued the publication. It is an alphanumeric system.

Therefore, information produced by the Department of Agriculture is found in the A's, Health and Human Services Department in HE and Congressional hearings will be found in the Y4s.

Here is an example of a SuDoc number:

 

The class number, before the colon, denotes the agency (Dept. of Commerce), subagency (3=Census), and the series (186). The report number indicates a unique publication within the series, and may consist of numbers or letters and numbers with punctuation. Punctuation includes colons, periods, slashes and dashes, all of which separate segments of the call number.  A more thorough introduction to SuDocs numbers can be found in  An Explanation of the Superintendent of Documents Classification System.