Understanding forms with collections

A form's collection may consist of one or more IDO specifications.

The first IDO specified returns the primary collection. Additional IDOs return secondary collections.

Secondary collections are useful when you need to create a form that works with multiple collections when some of those do not have parent-child relationships characteristic of subcollections. Since it is easier to implement forms with subcollections than forms with secondary collections, you should use secondary collections only in those cases where subcollections will not work.