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: