The class number, before the colon, denotes the agency (Dept. of Commerce), subagency (3=Census), and the series (186). The report number indicates a unique publication within the series, and may consist of numbers or letters and numbers with punctuation. Punctuation includes colons, periods, slashes and dashes, all of which separate segments of the call number. A more thorough introduction to SuDocs numbers can be found in An Explanation of the Superintendent of Documents Classification System.