Let us regard here data structures for long-term storing of data, as database schemata, but also tagging schemes like SGML/XML DTD, RDF Schema, and data structures designed as fill-in forms guiding users to a complete and consistent documentation [...]

These structures are always are compromise between the complexity of the information one would like to make accessible by formal queries, the complexity the user can handle, the complexity of the system the user can afford to implement or to pay for, the cost to learn those structures and to fill them with contents.

As most applications run in a relatively uniform environment – a library, a museum of modern art, a historical archive of administrational records, a paleontological museum etc., much of the complexity of the one application is negligible for another. This allows for a variety of simplifications, which are unavoidable to create efficient applications.