Patron-driven acquisition
Patron-driven acquisition (PDA), also referred to as demand-driven acquisition (DDA), is a model of library collection development in which a library only purchases materials when it is clear that a patron wants them. In an ideal transaction, libraries provide the patron with access to search engines, academic databases and/or library catalogs from which the patron can request items.
Source: Wikipedia — Patron-driven acquisition (CC BY-SA 4.0)