The major difference between the citation styles in the Chicago Manual of Style and the U.S. Government Printing Office (GPO) Style Manual are caused by the needs of the users of the two style guides.
The Chicago Manual of Style covers a much larger territory of printed material and therefore has a wider range of rules to cover writing and publishing of all types of books, periodicals and manuscripts. There are sections on capitalization and compounding but these are relatively minor sections compared to what's available on these subjects in the GPO Style Manual.
Much more attention is given to areas such as proper pagination of an article or manuscript, organization of the front and back matter of book and extensive coverage of punctuation.
A large portion of the Chicago Manual of Style covers the proper use of numbers in sentences and the expression of mathematics in the written word; this allows for proper treatment of mathematics in textbooks and reference works.
Both style manuals cover the citation of other works and their annotation in publication.
The GPO Style Manual contains several sections with commonly used reference material that would be of use to government workers involved in producing general federal documents. These references include:
A section unique to the Chicago Manual of Style in comparison to the GPO Style Manual concerns the treatment of illustrations and captions. This section is necessary due to the common usage of illustrations in many literary and reference works.
Illustrations are handled differently from normal printed material in terms of pagination, their entry in the list of references or in citing a piece of artwork or other visual piece in a written work.
The nature of printed government printed documents covered by the GPO Style Manual does not require a special treatment for these items.
Recording and publishing the Congressional Record is a section unique to the GPO Style Manual. This section contains comprehensive instructions for the proper recording of Congressional debates, hearings, votes and other business.
This detailed guidance is necessary both for accuracy in detailing the business of Congress and because the Congressional Record is a legal document that must be free of ambiguity or vagueness.
The GPO Style Manual covers both the proper format for recording of events and the methods of imprinting the physical copies of the record for proper identification. An adjunct to this section is a comprehensive listing of common government agency acronyms to ensure accuracy in the record.
The Chicago Manual of Style is a comprehensive reference work meant to fulfill the needs of writers, editors and publishers of books and periodicals. It is comprehensive and wide ranging due to large amount of material and differing formats of the general publishing industry.
The Government Printing Office Style Manual, on the other hand, is a reference work for a narrow field in publishing: government documents, reports and the Congressional Record.
Both manuals cover some of the same material, such as punctuation, document editing and word usage.
However, the GPO Style Manual places greater emphasis on being a reference work for government acronyms, general encyclopedic style information and producing the Congressional Record than on being a guide to writing and publishing.
The Chicago Manual of Style has limited informational sections, with the majority being devoted to the fine details of:
The Chicago Manual of Style and the Government Printing Office Style Manual serve the needs of two different authoring and publishing communities. The Chicago Manual is geared towards the commercial publishing market. The GPO Style Manual serves the needs of those publishing official government information and publications.