actionLib: difference between GoalCallbackMethod and ExecuteCallbackMethod

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Hi,

In the GoalCallbackMethod tutorial it says

If you need to see the goal before you accept it, see the SimpleActionServer(ExecuteCallbackMethod) tutorial.

I'm not quite clear what it means.

From my understanding, the executionCallbackMethod is for sending action request -> receive result, while the goalCallbackMethod is used for a a new parameter -> update result -> a new parameter -> update result ...

Am I correct? Is there any better explanation about the difference between these two methods?