Thanks
If you refer to “Reusable sub-process” as defined in BPMN 1.2, you need to use in BPMN 2.0 “Call activity”.
A call activity will trigger the execution of a target process (the “sub-process”).
A process called by a call activity is just a regular process in Bonita BPM. It has no specificities expect maybe that usually it does not have an actor flagged as “initiator”. In fact initiator is usually the execution engine running the call activity.