The SuDoc number for a publication is issued when the publication is distributed to depository libraries and is utilized by most depository libraries for shelving purposes and by all research finding aids to identify a particular publication.
The SuDoc number is based on issuing agency of a publication and its place the government organization. All publications of an issuing agency are categorized into series which are numbered.
The system utilizes letters, numbers and punctuation marks to create a unique number.
The number before the colon will place the publication in its series.
The number after the colon is what's assigned to an individual publication and completes the unique number.
Please note: In the SuDoc system, all the numbers are treated as whole numbers. There are no decimal numbers. The period in the SuDoc number is a period. It separates whole numbers and is not a decimal point.
Source: University of Rhode Island Libguide. http://uri.libguides.com/govpubus/sudoc