It all comes down to the EULA that I posted above
. When you by an upgrade copy, Microsoft gives you a price break by you not having to buy a more expensive "full" copy when you have a qualifying OS (ex: Windows 7) that you want to replace with Windows 8, and you agreeing to the EULA for that.
If you purchased an upgrade
copy of Windows 8, then you can do a clean install or upgrade install with it. However, to abide by the EULA, you must uninstall or upgrade from your say Windows 7.
If you purchased a system builder
copy of Windows 8, then you can install it and still be able to use your Windows 7.
Hope this helps to clear it up better.