If you are like me, you might spelunk from time to time in the bowels of your application's manifest file. But, "why?", you might ask, "Isn't everything in the manifest revealed in the graphical editor in Visual Studio?" Oh, you poor child. Allow me to enlighten you on this one: No.
One such setting is the <TargetDeviceFamily /> node, the child of <Dependencies />, the child of <Package />.
You can find it documented on MSDN where it provides this illuminating prose: "Identifies the device family that your package targets." There are some examples, sure, but I wanted to take a moment and make it painstakingly clear. I want to remove any doubt and leave you with the confidence to look at your manifest and edit it without fear.
Fact is, you set this when you create your project. The dialog above appears when you create a new Universal Windows Platform app. It's asking you the Target and Minimum version. But, of what? It is setting the initial versions of the "Windows.Universal" target device family.