Windows 8 System Builder version is an installation media for a computer that is not currently running Windows 7, Vista, or XP.
System Builder version can be activated according to version specification, e.g. "Windows 8 64-bit English 1pk DSP OEI DVD" can be activated only once (1pk).
If you are installing Windows 8 System Builder version in the manual mode – make sure you have selected the proper edition in the Windows 8 setup wizard – otherwise you will not be able to activate it (reinstallation will be required).
For more information on Windows installation please use KB 4729 "Installing Windows on your Mac using Parallels Desktop"