What is the difference between angular testing async+whenStable and fakeAsync+tick?

I understand that the async and fakeasync methods setup some kind of listener that records all async operations so that the angualar testing framework can use whenStable and tick to manage waiting for all that stuff to finish. I think thats correct?

The thing I am struggling to understand is whether there is actually an execution order difference - because if not why offer both?

This has brought me to learning about JS macro tasks and Micro tasks and I am wondering if this area is where the two approaches differ?