The various opaque types of Collection Services create objects that, to some degree, are containers of values. (In this document, the word "value" denotes an element contained by a collection.) But there are wide differences in how collections contain and dispense their values. The organization of the concepts in this section reflects these differences. What might be termed the "true collections"--arrays, dictionaries, sets, and bags--are described together because of their strong similarities. Then this section goes go on to describe trees, which are structurally quite different from the true collections. Finally, it summarizes the characteristics of data objects (of type CFData), which are collections in a much different sense than the other Collection Services objects.