In the dLibra system, the role of structural metadata is played primarily by group publications, which make it possible to create connections between elementary objects (publications) stored in the system. They allow the creation of complex objects, which represent newspapers, journals, multi-volume items, or groups of objects related in other ways. The creation of complex structure is also facilitated by collections, which allow the hierarchical organization of digital library resources. Collections are a more general mechanism because an object must belong to at least one collection, so every object can be found by browsing the collection structure on the digital library web pages.