Since both event is empty, there is no difference, you are going to get the same result which is nothing 
If there was conditions and actions in Group 1, both event would be tested and executed one after the other.
If there was conditions and actions in Group 2, the second event which is the child event of the first one would be tested and executed only if the first event was true and after it was executed.
If the first event was false and did not execute, the second event also not going to be tested and executed.
In case your question is what an empty event do, it is return true in both cases and move on to the next event.
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thank you so much!!!