Unfortunately, among its many other annoyances, the App Store does not clearly distinguish between apps designed specifically for and only for the iPad, those designed for all iOS devices, and those designed only for the iPhone/iPod. Even apps supposedly designed for all iOS devices, for example, may not automatically reorient the UI for landscape display, for example. In light of the absence of clear guidelines, the convention (but only that) is that apps with the designation "HD" are designed specifically for the iPad and are not meant to be used on the iPhone or iPod. It's confusing and there are exceptions but the rule seems to be more or less useful in searching for apps.