A DID (Database Interface Definition) is a description of (a part of) a database. ITP uses the abstract concept of Entries for files or tables. Relations between database entities are expressed through Entry-Subentry. Apart from Entries, DIDs can also contain Functions. A DID itself does nothing. It is used to create Models. During Model creation, the information that is needed is copied to the Model. When a Model is run ITP retrieves the data from the database, based on the information that is stored in the Model.