Calligra libs is a collection of libraries for very different functionality with the common denominator that some Calligra applications depend on it. Each of the libraries have a Mainpage.dox file which is used for API docs and it should contain an overview of the library goal and scope. If you add a class you should likely read this before deciding where it goes.
Various details can also be found here; community.kde.org/Calligra/Libs">http://community.kde.org/Calligra/Libs

SEE ALSO: README