It's pretty much all experience.. Generally, you probably don't want a datatype so specific that it will only capture one page, and you don't want to make it so generic that the whole site fits into a single datatype (yes, there are exceptions). Usually, you want to weigh the tradeoff between form complexity/usability and having content littered in so many different types that it can't be found. You do so by combining similar page templates into a single one with options (radio buttons and such), and if your datatype starts to look like an audio receiver, then you've gone too far.