Many units of the American Library Association have developed documents that give advice to librarians on various aspects of library service. These documents may be called "standards " or "guidelines". The ALA Standards Manual gives definitions of those terms, describes the authority of units to develop them, and includes advice on format and related sources. The categorized listing of ALA Standards & Guidelines includes documents entitled “standards” and “guidelines” as well as other documents of a similar nature entitled “statements,” “rules,” and “criteria.”