The Superintendent of Documents (SuDoc) classification number begins with a capital letter or letters representing a government department or agency. (A - Agriculture Dept., C - Commerce Dept., ED - Education Dept., etc.) The U. S. Congress uses the letters X and Y.
The numbers that follow these letters represent the office of a particular department. For instance, all Internal Revenue Service publications will be given a number beginning with T 22. The numbers after the decimal point represent types or series of publications, or second and subsequent level offices.
Numbers immediately following the colon indicate numbered series, volumes or dates. Letters and numbers immediately following the colon are based on a system which organizes materials alphabetically by subject or title keyword.
In general, documents are shelved in alphabetical and numerical order.
This is not a decimal system. Numbers between punctuation are treated as individual whole numbers.
Numbers precede letters.
For Senate Hearings and Prints for the 98th and subsequent Congresses, special shelving rules are needed. Documents are shelved (or filed) by S.HRG. (meaning Senate Hearing), then S.PRT. (meaning Senate Print) for each Congress by number. Here is an example sequence of these numbers:
Microfiche is filed in the same order as paper, in cabinets on the west wall
of the main floor.
The Government Printing Office, who assigns the SuDocs numbers to publications, provides this detailed Explanation of the Superintendent of Documents Classification System.