When creating apps for both Windows 8.1 and Windows Phone 8.1, you typically want roaming data–as managed through the WinRT APIs–to work seamlessly across both.

The first requirement here is that both apps are using the WinRT APIs for this purpose, meaning that the Phone app project is not using Silverlight but rather the "Windows XAML" or WinRT flavor. Clearly this isn't an issue for apps written in JavaScript, where that's the only option, nor for universal Windows apps that focus on WinRT.

Note that it's not necessary to implement the app with a universal app project in Visual Studio. Those project structures are simply a way to simplify development processes, and isn't a requirement for providing the same app experience on both Windows and Windows Phone.

The second requirement, however, is that both apps–however you build them–must have the same identify in their respective stores, which means specifically the package family name. This is what's used to manage the roaming settings, and so long as those match, then both implementations will work with the same roaming data. This is again a given with a universal app project, but something you have to do manually if implementing the apps with separate (non-universal) projects.

A little guidance on this can be found here: http://msdn.microsoft.com/en-us/library/windows/apps/xaml/dn630421.aspx

For some general Q&A on app data, which includes other aspects of roaming, see my post on the Building Apps for Windows blog: http://blogs.windows.com/buildingapps/2014/06/19/common-questions-and-answers-about-files-and-app-data-part-1-app-data/

With the introduction of universal Windows apps at //build 2014, and the announcement of Windows Phone 8.1 that includes support for writing apps in HTML, CSS, and JavaScript, we do have some resources that are starting to emerge.

First, my newly-released second edition, http://aka.ms/BrockschmidtBook2, is very applicable to Phone apps. I didn't have time to spell out all the differences, but there is a summary of what to watch out for in Chapter 1, namely a few differences in controls and some parts of WinRT that aren't on the Phone.

Second, watch the Building Apps for Windows Blog for material in this area. For example, an upcoming post will spell out the controls story between Windows (WinJS 2.0) and Windows Phone (WinJS 2.1).

Third, Josh Williams and Ryan Salva did a demo session on building a universal app with HTML, CSS, and JavaScript at //build, which you can find on http://channel9.msdn.com/Events/Build/2014/2-540. What you'll see is that they do nearly all of the work in the Shared folder of the VS project, making only one small change at the end of the session to switch the Hub control in the Windows Store app to Pivot in the Phone app…the two controls, in fact, are identical in their API except for the top-level object names, so you just change the names and voila! You're in business.

Finally, there is a post on the Visual Studio blog on universal apps: http://blogs.msdn.com/b/visualstudio/archive/2014/04/08/building-windows-phone-8-1-apps-in-html.aspx.

More is certainly to come.