Most other extensions have functionality (such as photo-editing, posting to social networks, etc) for which it makes sense to have a parent app.
But for the simplest of keyboard extensions, without any (user-facing) data management or other requirements, I don't understand what the parent app should do.
Just having a "Hello" splash screen seems like a cheat. But is this what Apple expects, at least when it comes to keyboard apps (the simplest ones)?
P.S:
Considering the documents are openly available here, I hope there won't be an NDA issue blocking this question.
I'm not sure if there is another SE sub-site on which this question is more appropriate. However, if necessary, I'd be glad to move the question.
I was looking for a keyboard-specific tag, but none seemed right. UIKeyboard seems specific to the earlier "default" iOS keyboard.