The collection property can be set from the shape menu as shown in the following figure.

collection property

Collection can be set on Data Item, Folder, Simple Data Type, Complex Data Type and Item Component.

elements with collection
Let’s have an item component that is composed with a depicted structure item component, setting the collection on the item component will set the collection on the Structured item component too, since they are the same. See the figure below.
item component structure collection