With the exception of some periodical titles, federal publications are arranged on the shelf by an alphanumeric system called the Superintendent of Documents Classification Scheme (SuDocs).
Letters stand for the publishing agency, for instance
A for Agriculture
C for Commerce
L for Labor
Pr for President
and so forth
The numbers on the top line of the call number file differently than the Dewey or LC collections. The number following the period is a whole number NOT A DECIMAL.
C 3.9 .nine files before
C 3.44 .forty-four, which files before
C 3.223 .two hundred-twenty-three