For the purpose of the sample app, I don't think it matters. We're basically defining a document type here, and we want to have a globally unique identifier for our app. However, this really only gets important when we begin distributing apps--and especially when we want to register our application to open that document type (for example, to allow people to email docs--or if other apps start using the same doc type). It won't actually affect iCloud performance, as far as I can tell. In fact, you can skip this step, and iCloud will work just fine.

Does that answer your question?