Directories are only meant to help editor order library publications. Logical structure seen by the reader is kept by the mechanism of library collections. In dLibra system there can be any number of collections and their structure is hierarchical. Every collection can contain any number of subcollections. There can be any number of publications that belong to a single collection, and each publication may belong to any number of different collections. Collections are in most cases created for a specific topic (eg. Educational materials and its subcollections Biology, Computer science).