https://www.activiti.org/userguide/#bpmnCallActivity

http://www.flowable.org/docs/userguide/index.html#bpmnCallActivity

https://docs.camunda.org/manual/7.4/reference/bpmn20/subprocesses/call-activity/

区别在于：camunda家支持Passing variables使用

<callActivity id="callSubProcess" calledElement="checkCreditProcess" >
  <extensionElements>
    <camunda:in variables="all" />
    <camunda:out variables="all" />
  </extensionElements>
</callActivity>

而activiti和flowable，都未提及支持。难道是camunda是规范，而activiti和flowable是实现？实现总是离规范有差距。

https://github.com/chanjarster/activiti-learn/wiki/详解call%20activity