Join Node or Merge Node?

Question

I am trying to make an activity diagram of a user for my system and I am unsure if the flow should come down to a Join Node or Merge Node before the user can log off. Here is the one I have made sa of now. Could anyone explain to me what's the difference?

Answer 1

It must be a join (though I first remembered wrongly and thanks to @AxelScheithauer pointing out my error). P. 401 of UML 2.5

15.5.3.1 Executable Nodes

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When an ExecutableNode completes an execution, the control token representing that execution is removed from the ExecutableNode and control tokens are offered on all outgoing ControlFlows of the ExecutableNode. That is, there is an implicit fork of the flow of control from the ExecutableNode to its outgoing ControlFlows.

That means that all 6 actions below will start in parallel which does not mean they must run concurrently. But all of them need completion in order to continue after the join below. (I doubt that this is desired.)

There's a (double) 2nd flaw in the top decision back-flows. They need to go back to the top merge. Otherwise neither Login nor Register would start since they expect 3 or 2 tokens where only one would arrive.