It denotes Inspection is enabled on that node. You can toggle it by pressing “A”
Ah right sorry, misread the question 
Unfortunately I have never encountered that arrow myself and can’t answer that question after all, If you do discover the reason please post it here, I too am now interested.
The only thing I can say is that it reminds me of the way blueprint represents breakpoints 
Bump
Seems like this arrows are related to hlsl nodes and GPU acceleration. The parts where point are processed with GPU are marked with arrow up and nodes where acceleration stops working are marked with arrow pointing down marker. But I’m not shure if I’m exacly right.
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