copy-file: ok-if-already-exists versus mustbenew

[Lennart Borgman]

I am trying to enhance my reading capabilities, but I can not understand 
the difference between the arguments ok-if-already-exists and mustbenew 
in the doc for `copy-file'. Can someone please explain this to me?

The only difference I see is that  "never overwrite" is mentioned for 
"mustbenew" - though I hope this holds for the first argument too. 
However it sounds like the second argument should be about modification 
times, but the doc string does not say so.