What is the relationship between process instance and execution

I’m learning about flowable recently, but I didn’t find the relationship between process instance and execution in the documentation. I would be grateful if someone could answer my question.

If I start a simple one-way process instance, Is there only one execution in this process instance? Under which circumstances will there be multiple executions of a process instance.

In java POJO, I see DelegateExecution has a method called getParentId(), I wanna know under what circumstances is its return value not null, and what does this id mean?

Thanks a lot if you can help with this

A process instance is an execution, a special one that is like a container and has a bunch of child executions.

No, two: one for the process instance and one for the actual execution path. The easiest is to check the ACT_RU_EXECUTION table when you’re stepping through a process instance.

A parent ID is set for example when using a boundary event. The execution waiting in the boundary event will have as parent id the execution in the activity on which the boundary event is.

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Thank you so much, it’s very helpful