The SuDocs (Superintendent of Documents) system is straightforward on the surface--A call numbers are publications of the Department of Agriculture, C is Commerce, D is Defense, and so on. This type of system is called a provenance system--it organizes publications based upon issuing agency.
There are exceptions to the straightforward letters, of course: Y 1 call numbers are general publications of Congress--House and Senate Documents and Reports, Y 3 call numbers are independent boards and commissions, and Y 4 call numbers are House and Senate Committee hearings and publications.