This step is about augmenting the model with external systems. An external system is defined as any system that is not a part of the domain being explored. It can execute commands (input) or can be notified about events (output).

The external systems are represented by pink sticky notes. In Figure 12-9, the CRM (external system) triggers execution of the “Ship Order” command. When the ship‐ ment is approved (event), it is communicated to the CRM (external system) through a policy.

Figure 12-9. External system triggering execution of a command (left) and approval of the event being communicated to the external system (right)

By the end of this step, all commands should either be executed by actors, triggered by policies, or called by external systems.