prochainement vs bientôt lesson 4.6

When should I use prochainement and bientôt? Any nuance or just for the purpose of learning new or another vocab of the same meaning? 

I think they are pretty much interchangeable.  I am not aware of any significant difference between the two of them.

Bientôt tends to be used with "positional" verbs, like arriver, partir, and aller, and the register is more casual/informal/langage courant. Prochainement has a more formal/official/langage soutenu register.

Thanks, it is ​just for my own understanding of the language. 

In the test I gave one answer, say prochainement (can't remember exactly which one without going back to the test) but didn't get any "correct" %. After several tries I went back to the lesson to "cheat" and used the other word and then I got 100%. It gave me the impression that the one I said was incorrect. I have no way of knowing if the answer was preset in the computer or that there was indeed a nuance that I didn't know about. 

 

torusan,

I didn't see your response until after I posted. I went back to the chapter and it was "prochainement" alone as a new vocab which I quickly forgot.  When I took the test bientôt came to mind when the word "soon" was presented. Now I hope I can retain all that nuances that you had pointed out so far.

Why do I need Cheryl Demharter when I have you? Haha! May be after I finished all 3 levels thoroughly and want to continue my own level 4...