The centralized systems model of IBM Rational Harmony

In the detailed architectural design phase you need to emphasize the definition of ports and interfaces, as well as arriving at the state-based behavior of the
sub-system blocks. In order to do so you have to identify the sub-system block ports and interfaces by using the white box sequence diagrams..

While the focus of the black-box activity diagram was to identify the different flows of system functions (operations), the white-box activity diagram focuses on collaboration between the different sub-systems while taking the allocation of the operations into consideration. The service requests that are received define the interfaces of a block. After ports and interfaces are defined, the resulting state-based behavior of each leaf block must be captured in a state chart diagram.

A delegate white box sequence diagram is shown in Figure 8. The sequence
diagrams present services that are requested from one sub-system block to the
other to satisfy the scenarios.