Complex sentence: Although sideloading apps from outside the store is supported, out-of-box sideloading support on Windows 8 is only available for Windows 8 "Enterprise" computers that have joined a Windows domain.
Split into simpler sentences: Although sideloading apps from outside the store is supported, not all computers running Windows 8 can do it. Out-of-box sideloading support is only available for Windows 8 Enterprise computers that have joined a Windows domain.