This article deals with the notion of documentary unit – a minimum informational unit, which is considered to be of interest of a group of users, liable to representation for the constitution of registration of a database – in order to explore the grounds of Cataloguing. Two conceptions have been taken into account: the concept of work proposed by Panizzi as part of the principles for the production of library catalogs, which was later resumed in the Functional Requirements for Bibliographic Records (FRBR) model; and the concept of subject as a way to distinguish the intellectual unit (from the physical unit), developed by Documentation and applied in systems of scientific information. It starts from the assumption that these conceptions frame themselves as conceptual-historical approaches to the notion of documentary unit, therefore making  it  belong  to its problematization.  As methodology,  we have carried  out conceptual-historical approaches of the two conceptions mentioned above, and an analyses about their present contribution. First, this article contextualizes Cataloguing, dealing with its aims and with the existing terminology, bearing in mind the processes and tools of production and management of database. Second, it presents some of the principles of Cataloging consolidated by Panizzi in the middle of 19th century, and the monographic principle proposed by Otlet for documentation based on those principles. Then, it discusses those principles and their applications along the 20th century. The article observes that the scenery portraited by these two conceptions, respectively under the predominance of the community of libraries, and the services and nets of scientific information, has taken new contours during the last two decades. It also shows that the concepts of work and subject are not self-exclusive aspects of activities of production and database management, but are general principles for the identification of a documentary unit from which the information registration is constructed.