Primo Ramdisk is available in four distributions: Standard, Professional, Ultimate and Server. If you want to run it on a Windows server OS, you must use the Server edition. If running on a Windows desktop OS, usually the Standard, Professional or Ultimate edition is used.
For a quick guide on how to choose the right edition, see
Which Edition Should I Choose?.
For detailed differences between these editions, see Primo Ramdisk Edition Feature Comparison.