UWP apps developed to work on [[smartphone]]s, [[personal computer]]s, [[video game console]]s and [[HoloLens]] are called '''universal apps'''. This is accomplished by using the universal app API, first introduced in Windows 8.1 and [[Windows Phone 8.1]]. [[Visual Studio 2013]] with Update 2 could be used to develop these apps.<ref>{{cite web|last1=WaheedGallo|first1=AhmedKevin|title=A first look at the Windows 10 universal app platform|url=httphttps://blogs.msdnwindows.com/b/msgulfcommunity/archivewindowsdeveloper/2015/03/02/a-first-look-at-the-windows-10-universal-app-platform.aspx/|website=MicrosoftWindows Gulf Technical CommunityDeveloper blogBlog|publisher=[[Microsoft]]|date=2 March 2015}}</ref><ref>{{cite journal|last1=Appel|first1=Rachel|title=Modern Apps : Build Universal Apps for the Windows Platform|url=https://msdnlearn.microsoft.com/en-us/archive/msdn-magazine/dn781364.aspx2014/september/modern-apps-build-universal-apps-for-the-windows-platform|website=[[MSDN Magazine]]|publisher=[[Microsoft]]|date=September 2014|volume=29|number=9|via=[[Microsoft Learn]]}}</ref> Windows 10 introduced [[Universal Windows Platform]] (UWP) 10 for developing universal apps. Apps that take advantage of this platform are developed with [[Visual Studio 2015]] or later. Older Metro-style apps for Windows 8.1, Windows Phone 8.1 or for both (universal 8.1) need modifications to migrate to this platform.<ref>{{cite web|title=Migrate apps to the Universal Windows Platform (UWP)|url=https://msdnlearn.microsoft.com/libraryen-us/mt148501.aspxprevious-versions/visualstudio/visual-studio-2015/misc/migrate-apps-to-the-universal-windows-platform-uwp|website=Archived Visual Studio Documentations|via=[[MSDNMicrosoft Learn]]|publisher=[[Microsoft]]|access-date=31 July 2015}}</ref><ref>{{cite web|title=Move from Windows Runtime 8.x to UWP|url=https://msdnlearn.microsoft.com/en-us/library/windows/appsuwp/xamlporting/mt238322.aspxw8x-to-uwp-root|websitework=UWP documentation|via=[[MSDN|Windows DeveloperMicrosoft CenterLearn]]|publisher=[[Microsoft]]|access-date=31 July 2015}}</ref>
UWP is not separate from Windows Runtime; rather, it is an extension of it. Universal apps no longer indicate having been written for a specific OS in their manifest; instead, they target one or more device families, e.g. desktop, mobile, console or [[Internet of Things]] (IoT). They react to the capabilities that become available to the device. A universal app may run on both a mobile phone and a tablet and provide a suitable experience. The universal app running on the mobile phone may start behaving the way it would on a tablet when the phone is connected to a monitor or a suitable [[docking station]].<ref>{{cite web|title=Guide to Universal Windows Platform (UWP) apps|url=https://msdn.microsoft.com/en-us/library/windows/apps/xaml/dn894631.aspx|website=[[MSDN|Windows Dev Center]]|publisher=[[Microsoft]]|access-date=31 July 2015}}</ref>
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'''APPX''' is the [[file format]] used to distribute and install apps on Windows 8.x, 10, 11, Windows Phone 8.1, Windows 10 Mobile, Xbox One, Xbox Series X|S, Hololens, and Windows 10 IoT Core.<ref>{{cite web|url=httphttps://msdnlearn.microsoft.com/en-us/libraryprevious-versions/windows/apps/hh464929(v=win.aspx10) |title=App packages and deployment (Windows Runtime apps) |websitework=[[MSDN|Windows Dev8 development |via=[[Microsoft CenterLearn]] |publisher=Microsoft |access-date=8 March 2016}}</ref> Unlike legacy desktop apps, APPX is the only installation system allowed for UWP apps. It replaces the [[XAP (file format)|XAP]] file format on [[Windows Phone 8.1]], in an attempt to unify the distribution of apps for Windows Phone and Windows 8.<ref>{{cite web |url=https://www.theverge.com/2014/2/11/5400660/windows-phone-8-1-features-leaked |title=Windows Phone 8.1 includes universal apps and lots of feature updates |first=Tom |last=Warren |date=11 February 2014 |publisher=[[Vox Media]]|work=[[The Verge]]}}</ref> APPX files are only compatible with Windows Phone 8.1 and later versions, and with Windows 8 and later versions.<ref>[http://www.wphoneapps.net/2013/12/how-to-install-xap-file-on-windows-phone.html How to Install APPX File on Windows Phone 8.1] {{webarchive |url=https://web.archive.org/web/20150425033027/http://www.wphoneapps.net/2013/12/how-to-install-xap-file-on-windows-phone.html |date=April 25, 2015 }}</ref>
The Windows Phone 8.x Marketplace allows users to download APPX files to an [[SD Card]] and install them manually. In contrast, sideloading is prohibited on Windows 8.x, unless the user has a developers license or in a business ___domain.<ref>{{cite web|url=http://www.howtogeek.com/129535/how-to-sideload-modern-apps-on-windows-8/ |title=How To Sideload Modern Apps on Windows 8 |work=How-To Geek}}</ref>{{Unreliable source?|date=March 2016}}
