Processes published on one process execution engine in one host can call processes or sub-processes published on a different process execution engine on another host. The called or remote processes are called external processes.
For example, a warehouse company may have a process that calls a sub-process that resides in a shipping company. Both processes are published and deployed on different process execution engines in different hosts and different networks. These are also typically behind firewalls. The warehouse process must be able to find the connection information for the shipping process in order to send the instance to the shipping process. An external process scenario could also include internal processes that are published and deployed on different process execution engine hosts.
When a process execution engine needs to communicate with a process located on another process execution engine host, it starts looking for an external process to establish the connection and reach the process. The external processes defined in the external process list are evaluated in descending order until an external process matching the required process information is found.