By Jared Bienz
Apps can specify that they prefer portrait or prefer landscape in their appxmanifest by checking one or more boxes:
However, the app is not guaranteed to get the preferred orientation. The classic example is a game that requests portrait but is being run on a desktop machine with a monitor that cannot be rotated. The game will be forced to run in landscape instead and it's up to the game to deal with this (e.g. letterboxing or putting content on the sides of the otherwise vertically laid out game).
At //build it was clearly stated that with 8.1 and the smaller 7" devices, most users prefer to hold the device in portrait. Of course in this scenario the device is portable and the OS knows that the device can be rotated, so if an app requests to run in landscape the OS would honor it. But be aware that it's still possible for an app to get locked in portrait too. It's less common but can happen. On my old desktop rig I ran dual 27" monitors where one was landscape and the other was portrait. On that machine if I ran store apps on the portrait monitor, the OS would force them to run in portrait.
In summary, you can ask for whatever orientation you want but be prepared not to get it.
Reprinted with permission.