In the pools they show their data personality. In the slots they show their operational personality. Anything that has only one personality has no use for pools or slots. Data may flow along paths and into and out of objects or slots, but it never becomes a slot occupant, it only flows to or from an existing object or slot. Data itself, as opposed to objects that store it, has no explicit existence in our design diagrams.
Data only has implicit existence as amorphous quantities hidden inside data objects.
When the design model includes a number of data objects, data elements may move or be copied from one of these objects to another, but these appear in the design diagrams of this book only as parameters that flow from place to place, not as explicit elements of defined data structures. We assumed this would be obvious to all, but discovered otherwise. Sec3.5.2 - This section does not distinguish between stub instances and stub types. It assumes all stubs are instances. See the comment on p168, Fig8.16 for a discussion of the difference between stub instances and stub types.