BPMN12 — Section: 9.4.2 Sub-Process

Hello, after reading throuh the named chapter about Sub-processes, I could not find a clear statement on the allowed number of "None Start Events" and "None End Events". Out of the spec one might have the feeling that a Sub-process always have a dedicated entry point from the "outside", which entry point is represented by a "None Start Event", and the sam epplies to End events/exit points. I could not find a statement regarding this, nor any overall principle out of which it could be concluded. It would maybe make sense to enrich the specification with this information.