In what situation would one use expectationForNotification in swift testing

I'm a little confused as what/when to do with expectationForNotification as opposed to expectationWithDescription. I've been unable to find any clear examples in swift for when and what you do with this call.

I'm assuming its perhaps to test notifications but it looks like it might just be a more convenient wrapper around the whole addObserver() call of notification center.

Could somebody give a brief explanation of what it does, when to use it, and perhaps a few lines of sample code?

Thanks