Sub-Process Start Events
There is only one type of Start Event for Sub-Processes in BPMN: None.
||The None Start Event is used for all Sub-Processes, either embedded
or called (reusable). Other types of triggers are not used for a
Sub-Process, since the flow of the Process (a token) from the parent
Process is the trigger of the Sub-Process. If the Sub-Process is called
(reusable) and has multiple Start Events, some of the other Start Events
MAY have triggers, but these Start Events would not be used in the
context of a Sub-Process. When the other Start Events are triggered,
they would instantiate top-level Processes.