U.S. federal government publications are arranged by the Superintendent of Documents (SuDocs) call number system. 

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), while others are not (e.g., X and Y 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, the category of publication (annual report, directory, bulletin, etc.), and information about the publication (year, issue, edition, etc., depending on publication type).

Note, SuDocs numbers do not read as a decimal, the way LC numbers do. For example, I 49.10 reads as forty-nine.ten. 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 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