Like a database and an application on the iSeries ITP's DID is essentially an abstract entity. Inside the ITP/SDK for iSeries the DID only exists as a coherent set of descriptions stored in several tables of the ITP/SDK for iSeries database. The DID actually becomes an object when it is downloaded to a PC for further use in creating and running ITP Models.
More than one iSeries library may be involved in one application and one library can even be part of more than one application, just like more than one DID-source may be linked to one DID and a DID-source may be linked to one ore more DIDs. A DID-source itself mostly (but not necessary) describes the data in one iSeries library.
A DID-source is less abstract than a DID in the aspect that a DID-source refers to actual objects in the form of one or more iSeries libraries. These libraries contain the sources and programs that may be needed to do the actual work involved in using Entries and User-defined functions when running an ITP Model.
Similar to (physical and logical) files and programs on the iSeries that are always located inside a library, Entries and User-defined functions can only exist inside a DID-source.
An Entry is mostly ITP's description of a logical or physical file: the Entry fields describe the fields in the file and the Formal parameters describe the key fields of the file. Unlike logical or physical files that are basically of a static nature an Entry also has a dynamic aspect. As an Entry is used for the specific purpose of retrieving data from a database when used in an ITP Model, it can also be considered to be the description of the interface of a program which performs the function "retrieve all information described by the Entry fields and identified by the Formal parameters from the database and return the results to the ITP Model".
A User-defined function is in many aspects similar to an Entry. The only differences are that a User-defined function has exactly one Result field and the User-defined function is intended to describe a program that performs some action other than the retrieval of data.
Entry fields and Formal parameters are ITP's equivalent to the fields and key fields of a (physical or logical) file or the result and input parameters of a program.