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.