A subprocess is an activity that contains other activities, gateways, events, etc., which itself forms a process that is part of a bigger process. A subprocess is completely defined inside a parent process (that’s why it’s often called an embedded Subprocess).

Subprocesses have two major use cases:

• Subprocesses allow hierarchical modeling. Many modeling tools allow that subprocesses can be collapsed, hiding all the details of the subprocess and displaying a high-level, end-to-end overview of the business process.
• A subprocess creates a new scope for events. Events that are thrown during execution of the subprocess can be caught by a boundary event on the boundary of the subprocess, thus creating a scope for that event, limited to the subprocess.

Using a subprocess does impose some constraints:

• A subprocess can only have one none start event, no other start event types are allowed. A subprocess must have at least one end event. Note that the BPMN 2.0 specification allows to omit the start and end events in a subprocess, but the current engine implementation does not support this.
• Sequence flows can not cross subprocess boundaries.

A subprocess is visualized as a typical activity, i.e., a rounded rectangle. In case the subprocess is collapsed, only the name and a plus-sign are displayed, giving a high-level overview of the process:

In case the subprocess is expanded, the steps of the subprocess are displayed within the subprocess boundaries:

One of the main reasons to use a subprocess is to define a scope for a certain event. The following process model shows this: both the investigate software and investigate hardware tasks need to be done in parallel, but both tasks need to be done within a certain time, before Level 2 support is consulted. Here, the scope of the timer (i.e., which activities must be done in time) is constrained by the subprocess.

A subprocess is defined by the subprocess element. All activities, gateways, events, etc. that are part of the subprocess need to be enclosed within this element.

<startEvent id="outerStartEvent" />
<!-- ... other elements ... -->
<subProcess id="subProcess">
   <startEvent id="subProcessStart" />
   <!-- ... other subprocess elements ... -->
   <endEvent id="subProcessEnd" />
</subProcess>

Camunda Extensions

Additional Resources

• Subprocesses in the BPMN 2.0 Modeling Reference