I was unclear what "Notification banners" mean, so I opened the documentation by hitting F1, but there's no section on Notifications. There are sections for other parts of the control center, there should probably be one for notifications as well.

control-center-3.16.2-1.fc22.x86_64


PS: I attach my IRC conversation with aday, he proposed a number of changes to the notifications dialog to improve clarity, so just in case you want to address it somehow:

<kparal> in the gnome-control-center -> notifications, what does Notification Banners mean?
<kparal> I don't understand the difference between Notifications on/off and Notification Banners on/off
<kparal> the help doesn't mention it
<aday_> kparal: i think we need some diagrams there :)
<aday_> "banners" == popups
<kparal> so what's the difference between notification and notification popup? :-)
<kparal> I thought that notification equals popup
<aday_> what do you mean by "notification"? which setting are you referring to?
<kparal> click g-c-c -> Notifications -> e.g. Firefox
<kparal> and then you have number of toggles
<kparal> the first one is called Notification, the third one Notification Banners
<aday_> ok
<aday_> if banners are off, but notifications are on, you will still get notifications in the drop down list
<kparal> ah, so they'll appear in the list, but they won't be "announced"
<aday_> right
<kparal> ok, thanks, that makes sense. I just couldn't figure it on my own
<aday_> maybe it would be better if we renamed "notification banners" to "show notification banners"
<aday_> or maybe we should rename banners to popups
<kparal> or add description to the F1 help page, currently there's no Notifications section at all. I'll report a bug about it
<kparal> btw popups would be clearer to me, but I'm not a native speaker
<aday_> we could also add descriptions to each of those switches
<kparal> or tooltips, that would work as well
<aday_> "allows popups to be hidden while still showing them in the list"
<aday_> kparal: i'd use a subtext rather than a tooltip