About a month ago, Microsoft announced that it would be releasing an ARM64 SDK at its Build 2018 developer conference. Today, the firm announced that the SDK is indeed available as part of the Visual Studio 15.8 preview.
Previously, developing a UWP app only allowed for developers to compile a package for x86, x64, and ARM32, as even 64-bit Windows phones used a 32-bit OS. Now, Windows 10 on ARM PCs run a 64-bit OS, so they can support the extra bits. Yes, these PCs can run emulated x86 apps, but those are also 32-bit.
After all, Windows 10 on ARM does not support x64 apps, and it never will, according to Microsoft. The performance just isn't there to support that. But the new SDK aims to solve that, because not only can developers use it to recompile their UWP apps as ARM64, but they can recompile their Win32 apps as well, and then they can use the Desktop App Converter to put it in the Microsoft Store if they want to.
As you can see from the image above, the SDK will still support both ARM32 and ARM64 for UWP apps. After all, the few remaining Windows phones will still not support these new ARM64 apps. Windows 10 Mobile isn't receiving feature updates anymore, so that will remain a 32-bit OS. If you don't plan on supporting phones though, you really don't have any need for 32-bit, since all Windows 10 on ARM PCs are 64-bit.
If you're still supporting a Windows 8 app since they run on Windows 10 too, that's the one thing that will not get ARM64 support. If this is the case, Microsoft recommends moving to UWP.
Microsoft said that in most cases, it takes minimal effort to recompile an app for ARM64. It talked about OpenVPN and VLC, saying that neither company had to change a single line of code in its applications. Of course, if an app uses third-party libraries, it might take some work.