DID Sources are used in the ITP/SDK for iSeries. Within a DID the Entries and Functions are defined in the DID Source. On the DID level you only specify which Main Entries and Functions are available to a Model Developer. A DID has one or more DID Sources. The DID Sources only have a meaning for the DID Developer. The Model Developer does not know anything about DID Sources and does not see a difference between Entries that are defined in different DID Sources.
You can link Entries together from different DID Sources through the Import-Export mechanism. With the ITP/SDK for iSeries you can only create DIDs for the ITP iSeries Connection. DID Sources do not have a connection attribute.