In general, all the documentation in a library is filed by document or sales number. However, subject files may be devised on issues for which information is often requested. The subject files can include, for example, duplicate copies of documents and press releases, pamphlets and material published outside the UN by organizations affiliated with the UN.

Subject files might be kept, for example, on the major UN conferences or on the budget of the UN. The subject files must be updated periodically in order to remain current.