The idea is to document a service that is built from a number of Components. Often this is enough to give a reader a good understanding what this service is about. Sometimes you want to drill deeper. Then you show the interfaces of a component in a Blackbox. You concentrate on the behavior that can be observed from the outside. Next you step into this component and document all the components on the next level inside this component detailed in the Blackbox. This documentation in the Whitebox document shows how the subcomponents work together inside the component.

In case you need to drill further into the components, you then describe each of the elements in the Whitebox View by a separate Blackbox document. Continue until everything you want communicate is in the alternating Blackbox / Whitebox documents.

Using the alternating Blackbox and Whitebox documents makes it easier to separate the documentation of the interaction of a component with its environment (Blackbox) from the documentation of the interaction of the subcomponents of the immediate next level.

If your design is not very deep, the documentation of a component may be enough. The Blackbox / Whitebox documentation simply allows to clearly communicate, what the focus of the individual document is.