It depends on the feature. For things that involve material changes to your app or a feature that you don't want Apple to be surprised about, usually you will i

It depends on the feature. For things that involve material changes to your app or a feature that you don't want Apple to be surprised about, usually you will include user credentials in your submission that have that feature enabled and give them a heads up. If's a smallish feature, you don't usually sweat app review. It's obviously not a perfect system.