When downloading versions of Unity, it’s pretty obvious what I need to BUILD for Mac, iOS and Android. They say “Build Support”.
But windows has two versions of “Store Backend” and nothing for “Build Support”.
What do I need if I’m going to create a Windows stand-alone build?
The Windows Build Support comes with the Windows version of Unity built-in.
How would you test your game without it 
Is that new? I always remembered having to choose all of the build support modules when using the standard installer, even Win on Win and Mac on Mac.
Interesting.
What the heck is Windows Store support and why do I have two choices?
For as long as I can remember it has always been this way. (Using Unity since 2012 but my memory isn’t so great so there’s that
)
The Windows Store just refers to the store for Windows 10, nothing special, I am not sure of the specifics of it having its own export option though. The 2 backends are .NET and IL2CPP. They are just different export options basically. IL2CPP does exactly what it says, it takes the IL from compiled C# and turns it into C++ usually for a basically free performance gain. However, IL2CPP apparently takes an age to build. IL2CPP has been present on mobile platforms for years now. (Since Unity 5 I think) as they require AOT (Ahead-of-time) compilation, C# is usually JIT (Just-in-time).