Well, I've downloaded their sample for "The Edge Screen" and what it does is:
Adds another "strip" to the edge area, and to see it, you have to sort of gently rub the screen where it curves (it takes me 3-5 attempts, maybe that's just me).
But before you scroll the edge area that far, to this particular sample app's "strip", on my S6 Edge I see a strip with the phone's current notifications, and AquaMail's was present there.
So I'm still quite confused by this feature. I do like having the clock and alarm indicator there, but the rest... Just not sure about how it works.
I'm also not sure if it's possible to for an app to force the edge area to show its notification specifically, i.e. skip the "rub the screen" part.
Oh, and the sample app showed in system settings under where you can select among the different -- very tall (long) -- previews of what your edge area might look like.
NOT where you have simple toggles for "notify in the edge area about missed calls / SMS / Email messages". That one would be better, I think,, but alas, seems to be reserved for Samsung's own apps...