# BPMN sub processes

A sub-process is an activity containing its own process. Collapsed, it keeps the parent diagram readable; expanded, it shows the detail.

It is the main tool for keeping a real process on one page.

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## When it matters

* Processes too large to read on one screen.
* Reusable fragments that appear in several processes.

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## The symbols

| **Symbol**            | **Means**                                                       |
| --------------------- | --------------------------------------------------------------- |
| Collapsed sub-process | Task with a plus marker. Detail lives elsewhere.                |
| Expanded sub-process  | Drawn inline in a box, for shorter detail.                      |
| Call activity         | Thick border. Calls a process defined separately, and reusable. |
| Event sub-process     | Dotted border. Triggered by an event rather than by flow.       |
| Transaction           | Double border. All or nothing, with compensation.               |

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## A few things that catch people out

* A call activity is reusable; an ordinary sub-process is not. The distinction matters when automating.
* Event sub-processes have no incoming sequence flow; they are triggered, not entered.
* Collapse anything that is not the subject of this diagram.

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## Related

[BPMN sequence and message flowsSolid lines stay inside; dashed lines cross.](https://help.gocapable.com/diagrams/bpmn-sequence-and-message-flows.html)

[BPMN pools and lanesWho is taking part, and who does what.](https://help.gocapable.com/diagrams/bpmn-pools-and-lanes.html)

[BPMN notation referenceBack to the full symbol reference.](https://help.gocapable.com/diagrams/bpmn-notation-reference.html)

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_Sequence flow stays inside the pool._